Add caching for the Mill build tool

[clintval]

Description:

Similar to Gradle, Maven, and sbt, I am hoping to see Mill cache support.

Mill is becoming a popular JVM build tool so I hope this is a welcome addition.

I have not contributed to this project before so please let me know if I missed anything!

Speaking of, how do I run automated tests on this branch?

Check list:

• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.

[Copilot]

Pull Request Overview

This PR adds caching support for the Mill build tool, expanding the action’s compatibility with popular JVM build tools.

• Adds Mill as a supported package manager with appropriate cache paths and file patterns in the source code.
• Introduces tests for Mill caching behaviors (error throwing, cache restoration consistency, etc.).
• Updates the documentation and end-to-end workflow to include examples and tests for Mill caching.

Reviewed Changes

Copilot reviewed 5 out of 9 changed files in this pull request and generated 1 comment.

File	Description
src/cache.ts	Adds Mill caching support to the PackageManager interface and supported array.
tests/cache.test.ts	Implements unit tests to verify Mill caching behavior.
README.md	Updates docs to include Mill as a supported package manager and usage examples.
.github/workflows/e2e-cache.yml	Adds e2e test workflow steps for saving/restoring Mill cache across OSes.

Files not reviewed (4)

• tests/cache/mill/.gitignore: Language not supported
• tests/cache/mill/.mill-version: Language not supported
• tests/cache/mill/build.sc: Language not supported
• tests/cache/mill/mill: Language not supported

[clintval]

I copied these patterns from above and notice a potential bug.

The matrix defines ubuntu-22.04 but the conditional here is still set to ubuntu-latest. I imagine this was not intentional. Is there a reason to pin to 22.04? If not, I'd vote reverting back to ubuntu-latest all over.