feat(vim.validate): improve fast form and deprecate spec form

Problem:

`vim.validate()` takes two forms when it only needs one.

Solution:

- Teach the fast form all the features of the spec form.
- Deprecate the spec form.
- General optimizations for both forms.
- Add a `message` argument which can be used alongside or in place
  of the `optional` argument.

runtime/lua/vim/treesitter/language.lua

@@ -154,10 +154,8 @@ end
 --- @param lang string Name of parser
 --- @param filetype string|string[] Filetype(s) to associate with lang
 function M.register(lang, filetype)
-  vim.validate({
-    lang = { lang, 'string' },
-    filetype = { filetype, { 'string', 'table' } },
-  })
+  vim.validate('lang', lang, 'string')
+  vim.validate('filetype', filetype, { 'string', 'table' })
 
   for _, f in ipairs(ensure_list(filetype)) do
     if f ~= '' then