Catch2 compatibility means the API tests are written in: the macros, such
as TEST_CASE and REQUIRE, and the Catch:: namespace. It does not mean
Catch2's build names or header paths. find_package(Catch2) found Catch3,
and a project using both frameworks met two packages, two sets of targets,
two Catch.cmake modules and two catch2/ header trees with the same names.

Build names:
- CMake: project and package Catch3, targets Catch3::Catch3 and
  Catch3::Catch3WithMain, libraries Catch3 and Catch3Main, options
  CATCH3_*, lib/cmake/Catch3 with Catch3Config, and pkg-config catch3 and
  catch3-with-main. The version is Catch3's, 0.3.0, compatible within a
  minor version.
- The CMake scripts are Catch3.cmake (catch3_discover_tests),
  Catch3AddTests.cmake and Catch3ShardTests.cmake
  (catch3_add_sharded_tests); the helpers they define at ctest time have a
  catch3_ prefix. ParseAndAddCatchTests.cmake, deprecated upstream, is gone.
- Bazel //:catch3 and //:catch3_main, Meson catch3 and catch3-with-main
  (with override_dependency), Xmake catch3 and catch3_main, Premake Catch3
  and Catch3WithMain. Visual Studio projects get GUIDs of their own.
- The Progmasoft::Catch3 target and package are gone: property testing
  and snapshots are part of Catch3::Catch3. Their sources now build with
  the library's warning flags, so property failures are built field by
  field (no missing initializers) and the default property name is a
  thread-local pointer to the name its scope owns (no global destructor).

Headers: the engine moved from src/catch2 to src/Progmasoft/Catch3 with
Progmasoft names, so <catch2/catch_test_macros.hpp> is
<Progmasoft/Catch3/TestMacros.hpp>, <catch2/matchers/catch_matchers_string.hpp>
is <Progmasoft/Catch3/Matchers/String.hpp>, and interface headers are named
after their interface, such as Interfaces/IConfig.hpp. Include guards
became #pragma once. The installed headers go to include/Progmasoft.
src/Progmasoft/Context.cpp is StrategyContext.cpp, so no two sources share
a file name.

tools/scripts/upstreamFiles.txt lists the files that come from Catch2.
The license check expects BSL-1.0 on them and Progmasoft's header on the
rest of src/Progmasoft; the convenience-header check, the amalgamation
script and the surrogate tests take the engine's headers from it. The
amalgamated files are regenerated. The header-guard workflow checks for
#pragma once instead of checkguard.

Documents, the README and the release notes describe the new names.

Catch3 keeps Catch2's API for writing tests (test cases, sections,
assertions, matchers, generators, listeners, benchmarks) and replaces its
session, command line and reporters with a runner of its own.

The runner starts the test binary again as worker processes that take the
next test when they are free. A crash, an exit or a hang ends one test, not
the run. On top of that it adds:

- retries that tell deterministic, flaky and order-dependent failures
  apart, and a bisection that names the test leaving state behind;
- a history of verdicts and durations, kept under a file lock, that drives
  automatic quarantine, adaptive time limits, predictive ordering,
  incremental runs, failure replay and analytics;
- selection by change, past failure or prediction, dependency closure,
  and sharding by a stable hash or by recorded durations;
- tags for serial and isolated tests, shared resources, dependencies,
  time limits and retries;
- a failure category, a signature, an analysis and a replay command for
  every failure;
- console, JUnit XML (with flakyFailure and rerunFailure), JSON and Chrome
  trace reports, and a versioned discovery listing (--list=json);
- a temporary directory per test (TMPDIR, TMP, TEMP and
  TestTemporaryDirectory());
- Bazel's test environment: XML output file, test filter, sharding, seed,
  premature exit file and undeclared outputs.

The command line is parsed by a hand-written, typed parser. Catch::Session
remains as a thin facade over the runner for custom main functions.

The result model in Progmasoft/Catch3/Results.hpp was rewritten around
tests and attempts, and the JUnit writer now writes it. The property
engine's example database is updated under a file lock with an atomic
rename, so parallel workers can write it at the same time.

Removed: Clara, the command-line and reporter-spec parsers, every reporter
and CATCH_REGISTER_REPORTER, listing, text flow, console colour and width,
the JSON and XML writers, sharding in the test registry, approval tests and
their baselines, Conan packaging, and the unused third_party/clara.hpp.
Listeners keep working; reporters' list and skip events are gone.

The Catch2 library is now compiled as C++20; code that includes only
Catch2's headers still needs C++14. Duplicate test cases are still refused
at start-up. Version 0.3.0.

ExtraTests now turn exceptions off for clang-cl with MSVC-style options,
which -fno-exceptions did not do.

Closes #2149

There were 2 separate bugs, one recently added and one ancient
(since roughly the first version of the registration script).

1) The recent refactoring of how the JSON output from Catch2 is
   parsed caused shadowing between the variable containing the
   per-test JSON fragments and the accumulator of test names for
   <target>_TESTS variable. This led to the variable containing
   both the names and the JSON fragments, and thus being completely
   wrong.

2) The test name accumulation has never accounted for characters
   that need escaping to be present in a CMake list. This means
   that e.g. test names with semicolon in them would end up with
   two elements in the test list.

Both of these are now fixed, at the cost of extra complexity and
my sanity as I had to learn more about CMake escaping rules.

(CMake escaping rules are dumb)

This addresses https://github.com/catchorg/Catch2/issues/2411 by making the "workaround" the official approach and is consistent with how gtest and cmake deal with the same situation.

This allows it to deal with badly behaved code, where 3rd party
dependencies write into stdout during global construction.

Closes #3162
Closes #3166

2 spaces seems to be the more common indentation level so that's what
I unified around.

We also bump the minimum CMake version to 3.20 as per #2943

I also added `SKIP_IS_FAILURE` option to the `catch_discover_tests`
function, to allow users to get back the old behaviour.

Closes #2873