Mark __CLASS__ as experimental

[rwp0]

Fixes #22288

@leonerd please consider reviewing this if you can

[rwp0]

I guess I should also add an entry (to document the new warning) in perldiag.pod as with ADJUST for instance?

[rwp0]

Feedback:
Would like to create/use a label to group all class related PR's together:

[rwp0]

# Failed test 1939 - __CLASS__ is experimental at porting/diag.t line 567
#     Message '__CLASS__ is experimental'
#     from toke.c line 8007 is not listed in pod/perldiag.pod
porting/diag.t ............ 
Failed 1/2[30](https://github.com/Perl/perl5/actions/runs/9864703324/job/27240172900?pr=22387#step:12:31)2 subtests

OK, hello diag.t:

      } else {
        # No listing found, and no excuse either.
        # Find the correct place in perldiag.pod, and add a stanza beginning =item $name.
        fail($name);
        diag("    Message '$name'\n    from $codefn line $. is not listed in $pod");
      }

🙂

[leonerd]

LGTM