Also split out helpers for testing matcher ranges (types whose
begin/end/empty/etc require ADL lookup, types whose iteration
uses iterator + sentinel pair, etc) into their own file.

* Add new SKIP macro for skipping tests at runtime

This adds a new `SKIP` macro for dynamically skipping tests at runtime.
The "skipped" status of a test case is treated as a first-class citizen,
like "succeeded" or "failed", and is reported with a new color on the
console.

* Don't show "skipped assertions" in console/compact reporters

Also extend skip tests to cover a few more use cases.

* Return exit code 4 if all test cases are skipped

* Use LightGrey for the skip colour

This isn't great, but is better than the deep blue that was borderline
invisible on dark backgrounds. The fix is to redo the colouring
a bit, including introducing light-blue that is actually visible.

* Add support for explicit skips in all reporters

* --allow-running-no-tests also allows all tests to be skipped

* Add docs for SKIP macro, deprecate IEventListener::skipTest

Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

This has two effects:
 1) This significantly improves the performance of ApprovalTests
    run in WSL, because running the massively multi-reporter
    test would cause lots of small and parallel writes across the
    WSL-Windows FS boundary, completely mudering performance.

 2) ApprovalTests can be run from multiple build dirs in parallel,
    as long as they do not fail. However, if they do fail,
    the multiple runs will still step on each other toes when
    writing the unapproved files for user.

This is primarily done to support new `std::*_ordering` types,
but the refactoring also supports any other type with this
property.

The compilation overhead is surprisingly low. Testing it with
clang on a Linux machine, compiling our SelfTest project takes
only 2-3% longer with these changes than it takes otherwise.

Closes #2555