We currently have logic spread in multiple places that normalize `nxtP`
parameters into their regular names, ie `nxtPfoo` -> `foo`. However we
don't apply this same logic to `nxtI`, which conceptually is another
parameter but for route interception. As a result, the interception
route would 404 when deployed because it'd be missing the interception
parameter.
This moves the normalization check into a util and updates the spots I
could find where we handle `nxtP` to also handle `nxtI`.
### Test Plan
Added a new test, and validated via these deploys:
**Working**:
[Link](https://vtest314-e2e-tests-m889gxi4p-vtest314-ijjk-testing.vercel.app/)
**Non-Working**:
[Link](https://vtest314-e2e-tests-8sa5t9uau-vtest314-ijjk-testing.vercel.app/)
Fixes#62207
Closes NEXT-3204
Sebastian "Sebbie" Silbermann