[Feature #19538]

This new `peformance` warning category is disabled by default.
It needs to be specifically enabled via `-W:performance` or `Warning[:performance] = true`

This fails on RubyCI due to RJIT warnings too often. It should be enough
to test RJIT feature in test_rjit_version. We only want warnings instead
of test failures when there are warnings, at least for this case.

This is useful for crash triaging. It also helps to hint extension
developers about the misuse of `rb_thread_call_without_gvl()`.

Example:

    $ ./miniruby -e 'Ractor.new{Ractor.receive};
        Thread.new{sleep}; Process.kill:SEGV,Process.pid'
    <snip>
    -- Threading information ---------------------------------------------------
    Total ractor count: 2
    Ruby thread count for this ractor: 2

for consistency