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/termcap.lua

@@ -17,10 +17,8 @@ local M = {}
 ---           otherwise. {seq} is the control sequence for the capability if found, or nil for
 ---           boolean capabilities.
 function M.query(caps, cb)
-  vim.validate({
-    caps = { caps, { 'string', 'table' } },
-    cb = { cb, 'f' },
-  })
+  vim.validate('caps', caps, { 'string', 'table' })
+  vim.validate('cb', cb, 'function')
 
   if type(caps) ~= 'table' then
     caps = { caps }