Note that only documentation and comments are impacted by this change.

Parsing C++ with regex in CMake is error prone and regularly leads to silently
dropped (not run) test cases.

Going forward the function `catch_discover_tests` from `contrib/CMake.cmake`
should be used.

For more information see https://github.com/catchorg/Catch2/issues/2092#issuecomment-747342765

Set `_CATCH_DISCOVER_TESTS_SCRIPT` helper variable globally. Otherwise in a
scoped call (like `add_subdirectory()`) the variable gets lost. This lost
variable results in a post build error with not much information to lead to the
root of the problem.

This enables the usage of the helper script with the following example structure

- CMakeLists.txt (project root with `add_subdirectory(external/catch2)`
- external/catch2
  - CMakeLists.txt (contents listed below)
  - contrib/Catch.cmake
  - contrib/CatchAddTests.cmake
  - catch2/catch.hpp
- tests
  - CMakeLists.txt (add tests with `catch_discover_tests(${PROJECT_NAME})`)

contents of project specific helper `external/catch2/CMakeLists.txt`
```cmake
cmake_minimum_required (VERSION 3.1...${CMAKE_VERSION})
project(Catch2 LANGUAGES CXX VERSION 2.13.3)
add_library(Catch2 INTERFACE)
target_include_directories(Catch2
  INTERFACE
    $<BUILD_INTERFACE:${CMAKE_CURRENT_LIST_DIR}>
)
 # provide a namespaced alias for clients to 'link' against if catch is included as a sub-project
add_library(Catch2::Catch2 ALIAS Catch2)
include(contrib/Catch.cmake)
```

CMake 3.19 introduces new add_test() behavior guarded with the policy
CMP0110.

See: https://cmake.org/cmake/help/latest/policy/CMP0110.html

Update the helper script ParseAndAddCatchTests to consider the policy and
handle the test case name accordingly.

Consistently use 4 spaces instead of tabs

Fix regex that requires two string arguments in the form of
TEST_CASE("a", "b") resulting in not finding TEST_CASE("a") entries.

See https://regex101.com/r/JygOND/1

Fixes: https://github.com/catchorg/Catch2/issues/2055

* Change regex to allow parentheses inside the test macro for a type list
* Append a wildcard to the CTestName if the test case is a template
* Also change the regular expression so parentheses are allowed in names
  (fixes #1848)