Add Ftreesit_grammar_location

Add Ftreesit_grammar_location

* src/treesit.c (treesit_loaded_lang): New struct.
(treesit_load_language): Return a struct instead of just the language
object.  The struct contains both the language object and the path to
the shared library.
(Ftreesit_language_available_p, Ftreesit_language_abi_version)
(treesit_ensure_query_compiled, Ftreesit_parser_create): Update
call of treesit_load_language.
(Ftreesit_grammar_location): New function.

(cherry picked from commit 2a75693f24a631b0a2a366bdce1ca5cb0c38d215)