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/_watch.lua

@@ -59,9 +59,9 @@ end
 --- @param callback vim._watch.Callback Callback for new events
 --- @return fun() cancel Stops the watcher
 function M.watch(path, opts, callback)
-  vim.validate('path', path, 'string', false)
+  vim.validate('path', path, 'string')
   vim.validate('opts', opts, 'table', true)
-  vim.validate('callback', callback, 'function', false)
+  vim.validate('callback', callback, 'function')
 
   opts = opts or {}
 
@@ -125,9 +125,9 @@ end
 --- @param callback vim._watch.Callback Callback for new events
 --- @return fun() cancel Stops the watcher
 function M.watchdirs(path, opts, callback)
-  vim.validate('path', path, 'string', false)
+  vim.validate('path', path, 'string')
   vim.validate('opts', opts, 'table', true)
-  vim.validate('callback', callback, 'function', false)
+  vim.validate('callback', callback, 'function')
 
   opts = opts or {}
   local debounce = opts.debounce or 500