refactor(snack-runtime): split runtime app into reusable package and app

[byCedric]

Why

This allows some of our partners to reuse the Snack infrastructure to run Snacks.

How

• [x] Copied the current /runtime code to /packages/snack-runtime
• [x] Moved the required changes from the custom runtime fork
• [x] Updated our CI for snack-runtime
• [ ] Cleaned up the old /runtime app to use the /packages/snack-runtime app → Postponing in a separate PR
• [x] Published and replaced all file:../ references in dependencies
• [x] Added context-based API to set the aliased modules for snack-runtime

Test Plan

TBD

[byCedric]

Tested this in https://github.com/byCedric/snack-custom-runtime, and it seems good to go!