Register migrateImport to ensure it actually runs

[adamwathan]

This PR makes sure the migrateImport codemod is properly registered so that it runs as part of the upgrade process.

Test plan

This PR adds a new v3 playground with an upgrade script that you can use to run the upgrade from the local package. When you add a non-prefixed @import to the v3 example, the paths are now properly updated with no errors logged:

Screen.Recording.2024-10-24.at.13.28.03.mov

[adamwathan]

When testing this manually (when we finally figured out how to do that locally, spoiler this was very hard), we noticed a bunch of warnings still that aren't surfaced when running the tests, because our analyze step runs before we rewrite the imports.

So this isn't actually ready to go yet, we're going to want to resolve that before merging.

It still doesn't work 😄

[philipp-spiess]

@adamwathan Pushed two changes onto this branch just now:

1. Fix the error we were seeing where the CSS module resolver we built ourselves to find the CSS root couldn't find relative files without the ./ syntax.
2. Add a v3 playground with a script that patches some dependencies so that we can properly test the upgrade scripts locally without much hassle. Included a small video in the PR description that showcases how you would use this to verify this fixes the issue.