Make C-extension Ractor-safe

[mohamedhafez]

In planning ahead to the near future when TruffleRuby can run C-extensions marked with rb_ext_ractor_safe(true) in parallel, and for when Ractors are no longer just experimental, it would be great if the C-extension could be made Ractor-safe, or marked as such if it already is so! Would be happy to send in a PR in the later case.

[eregon]

From a quick look at

rbs/ext/rbs_extension/main.c

Lines 4 to 9 in c255458

	Init_rbs_extension(void)
	{
	rbs__init_constants();
	rbs__init_location();
	rbs__init_parser();
	}

and the 3 callees it looks fine.

rbs/ext/rbs_extension/parser.c

Lines 2985 to 2987 in c255458

	VALUE empty_array = rb_obj_freeze(rb_ary_new());
	rb_gc_register_mark_object(empty_array);
	EMPTY_ARRAY = empty_array;

is fine because it's an immutable/deeply-frozen object.
https://github.com/ruby/rbs/blob/master/ext/rbs_extension/constants.c are just some written-once variables (so basically constants).
And https://github.com/ruby/rbs/blob/master/ext/rbs_extension/location.c seems to have no state as well.

@mohamedhafez Could you make a PR?