ZJIT: Support enabling ZJIT lazily with RubyVM::ZJIT.enable

[k0kubun]

We should have RubyVM::ZJIT.enable that allows a Ruby process that started with JIT disabled to lazily enable ZJIT. It allows to skip wasting the code region for boot time-only code, also avoiding unnecessary compilation overhead on warmup.

The patch should look similar to RubyVM::YJIT.enable ruby#8705. The PR already took care of the cont.c changes needed for any JIT, so you shouldn't need to touch the file this time.

1. You need to update zjit/src/state.rs a little to make sure boot-time initialization is called at boot even if ZJIT is disabled.
   • cruby_methods::init() should be called at boot, not at RubyVM::ZJIT.enable. cmes could have been re-defined after boot.
2. Add RubyVM::ZJIT.enable to zjit.rb.
   • Use Primitive to call a Rust function like yjit.rb does. It should do the current rb_zjit_init() but without (1).
   • Similarly to ruby_set_yjit_description, you need to define ruby_set_zjit_description in version.c. The Rust function should call it too.

[chancancode]

@k0kubun I was poking around the ZJIT code last night, figured this would serve as a good orientation for the codebase, took a stab at implementing it in ruby/ruby#15219

[k0kubun]

cool, thanks!