Warn users and revert their `devtool` when they manually change the `devtool` in development mode. For this addition, I check to ensure the `devtool` is custom (i.e. different than what is set by Next) and has a value (`false` is fine as a custom `devtool`!).
As described in [this issue (13963)](https://github.com/vercel/next.js/issues/13963), changing the `devtool` in development mode can cause issues with performance.
Fixes#13963
