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/command_spec.lua

@@ -543,7 +543,7 @@ describe('nvim_create_user_command', function()
   end)
 
   it('does not allow invalid command names', function()
-    matches("'name' must begin with an uppercase letter", pcall_err(exec_lua, [[
+    matches("Invalid command name %(must begin with an uppercase letter%): 'test'", pcall_err(exec_lua, [[
       vim.api.nvim_create_user_command('test', 'echo "hi"', {})
     ]]))