Fixes #2615

Co-authored-by: Garz4 <fancygarz4@gmail.com>
Co-authored-by: Martin Hořeňovský <martin.horenovsky@gmail.com>

* 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>

They also got slapped with the `[approvals]` tag in the process,
because we have too many approval tests and want less of them,
and these particular tests don't bring much value.

Related to #2090

This changes the compact reporter's summary of test run totals to use
the same format as the console reporter. This means that while output is
no longer on a single line (two instead), it now includes totals for
`failedButOk` test cases and assertions, which were previously missing.

This worsens the message for negative numbers a bit, but
simplifies the code enough that this is still a win.

There is an increasing number of places where Catch2 wants to parse
strings into numbers, but being stuck in C++14 world, we do not
have good stdlib facilities to do this (`strtoul` and `stoul`
are both bad).