This is _technically_ a performance improvement, but in actuality
it is a tiny cleanup so it stops bothering me.

By avoiding `std::set`, we save 1 allocation per checked test case,
and the sort + pairwise check has much nicer cache behaviour, leading
to much faster check.

Overall, the check is only a tiny part of the startup overhead, but
the improvement is large enough that it ends up measurable in binaries
with many tests (e.g. `SelfTest`).

We used to store a full vector of "views" of the registered test infos,
so that it can be retrieved externally. However, the only user of that
interface left was `Session`, specifically `Session::applyFilenameAsTags`.

By moving the adding of the filename tags into the `ITestCaseRegistry`
interface, we avoid storing the vector and make the initial test registration
faster.

The new code is ugly and shows a design/layering issue, but no worse than
the previous approach did.

Preallocating to some reasonable and small number of tests avoids
much of the geometric-reallocation threashing at low sizes, without
taking up too much memory for tiny test binaries.

This fixes compilation issue with C++23 mode against libstdc++.

Closes #2852

Mainly just removes some unused includes, but sometimes the include
is replaced by a smaller header instead.

Closes #2682

* Add new `CATCH_CONFIG` option for using `std::getenv`, because PS does not support env vars
* Add PS to platforms that have disabled posix signals.
* Small workaround for PS toolchain bug that prevents it from compiling `std::set` with lambda based comparator.

The license file was renamed with 6a502cc2f5fedd59b3495b58708f0d6d987ed9e1