refactor(api): VALIDATE macros #22187

Problem:
- API validation involves too much boilerplate.
- API validation errors are not consistently worded.

Solution:
Introduce some macros. Currently these are clumsy, but they at least
help with consistency and avoid some nesting.

test/functional/api/buffer_updates_spec.lua

@@ -762,7 +762,7 @@ describe('API: buffer events:', function()
   it('returns a proper error on nonempty options dict', function()
     clear()
     local b = editoriginal(false)
-    eq("unexpected key: builtin", pcall_err(buffer, 'attach', b, false, {builtin="asfd"}))
+    eq("Invalid key: 'builtin'", pcall_err(buffer, 'attach', b, false, {builtin="asfd"}))
   end)
 
   it('nvim_buf_attach returns response after delay #8634', function()