Associate user-defined magic to variables from Perl.

Changes for 0.64 - 2024-03-18T23:20:00Z

• This is a maintenance release. The code contains no functional change. Satisfied users of version 0.63 can skip this update.
• Add : Contributing guidelines are now listed in the new CONTRIBUTING file.
• Fix : [RT #151104] : fix for t/18-opinfo.t broken under blead Some optimization in core made t/18-opinfo.t fail since perl 5.39.7, but that was reverted before 5.40 was released. This fix will make this test pass even when the optimization is reinstantiated after release freeze. Thanks David Mitchell for reporting and providing a fix.