feat(treesitter)!: use return values in language.add()

Problem: No clear way to check whether parsers are available for a given
language.

Solution: Make `language.add()` return `true` if a parser was
successfully added and `nil` otherwise. Use explicit `assert` instead of
relying on thrown errors.

runtime/lua/vim/treesitter/query.lua

@@ -247,8 +247,7 @@ end)
 ---
 ---@see [vim.treesitter.query.get()]
 M.parse = memoize('concat-2', function(lang, query)
-  language.add(lang)
-
+  assert(language.add(lang))
   local ts_query = vim._ts_parse_query(lang, query)
   return Query.new(lang, ts_query)
 end)