fix(dart_frog_cli): disallow middleware of middleware #774

Description

Currently you can make a middleware of a middleware. I think this is not intended, but if it is, feel free to close the issue.

Steps To Reproduce

1. Create a Dart Frog project:

dart_frog create sample

2. Create a middleware (from the root of the Dart Frog project):

dart_frog new middleware /

3. Create a middleware of the middleware (from the root of the Dart Frog project):

dart_frog new _middleware

Expected Behavior
To get an error message when creating the middleware of a middleware.

Screenshots
Result after doing the steps detailed above: