Add feature flag for new dev overlay (#73977)
This PR enabled feature flag for the new dev overlay, no behavioral
change.
- Added `experimental.newDevOverlay` Next.js option, which handles
`__NEXT_EXPERIMENTAL_NEW_DEV_OVERLAY` env value.
- Added `__NEXT_EXPERIMENTAL_NEW_DEV_OVERLAY` env to be resolved at
build time so that it can be used to render the overlay conditionally
