`next-build` should warn about existing `.next`

[rauchg]

This way, people can avoid accidents and side-effects. Typically if you're starting a next build you want to start from a clean slate.

Displaying a simple warning in stdout is probably a good idea.

[nkzawa]

It removes .next directory before starting the actual build process. I think it's better than displaying a warning.

[timneutkens]

@rauchg @nkzawa Think this can be closed 🤔

[nkzawa]

@timneutkens 👍

Let's reopen if we need.