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/func/_memoize.lua

@@ -39,10 +39,8 @@ end
 --- @param strong? boolean
 --- @return F
 return function(hash, fn, strong)
-  vim.validate({
-    hash = { hash, { 'number', 'string', 'function' } },
-    fn = { fn, 'function' },
-  })
+  vim.validate('hash', hash, { 'number', 'string', 'function' })
+  vim.validate('fn', fn, 'function')
 
   ---@type table<any,table<any,any>>
   local cache = {}