chore: add test mapping script (#63757)
### What?
This adds a script that first gets a list of changed files compared to
our base branch (`canary`) in PRs and identifies related test files by
looking them up from a `related-tests-manifest.json`, then runs them.
### Why?
We would like to be able to run related E2E tests when certain files
change, so we can test deployment stability.
NOTE: We still want to run all E2E tests on PRs, but this script can be
used to also test deployments for hot paths. Since these tests can take
up to [3-4
hours](https://github.com/vercel/next.js/actions/workflows/test_e2e_deploy.yml),
we aim to run a subset of these tests first.
Closes NEXT-2947
