Fix invalid source maps from terser-webpack-plugin (#45423)
This fixes a regression in our source map generating which got lost in
the big diff from the `src` folder restructure in
https://github.com/vercel/next.js/pull/44405
These invalid source maps broken plugins that attempted to leverage them
like `@sentry/nextjs` which only attempts in a production environment
https://github.com/getsentry/sentry-javascript/blob/15ec85bead77100413381be821f42841ae114f93/packages/nextjs/src/config/webpack.ts#L586
For a regression test in a follow-up we will need to investigate a
production test fixture with `@sentry/nextjs` although this requires a
DSN be configured.
This also ensures we setup `unhandledRejection` and `uncaughtException`
listeners during build so that we have proper stack information when
these occur and the process isn't left hanging.
This also moves the `extensionAlias` config from
https://github.com/vercel/next.js/pull/44177 to an experimental config
as it seems to cause conflicts with ESM packages that define `exports`
in their `package.json` which can be considered a breaking change.
## Bug
- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
Fixes: https://github.com/vercel/next.js/issues/45419
x-ref: [slack
thread](https://vercel.slack.com/archives/C03DQ3QFV7C/p1674937545579229)
