Print error when next dev has conflicting app & page (#41656)
This PR prints an error when the `app` and `pages` directory contain the
same route.
## build error
<img width="904" alt="image"
src="https://user-images.githubusercontent.com/229881/197424839-67dac580-1e1d-4c31-b769-112f2f38b06e.png">
## dev error
<img width="957" alt="image"
src="https://user-images.githubusercontent.com/229881/197426784-9b5041d9-b6d0-48d5-8ce5-b759bd9e4438.png">
Co-authored-by: JJ Kasper <jj@jjsweb.site>
