fix: Avoid re-running build.rs on every build, even when nothing changed

[refi64]

Issue # (if available)

Description of changes

from commit message:

Because build.rs depends on the bundle/ directory, whenever it also writes into it, the directory's timestamp is updated. This leads Cargo to always consider it outdated and re-run build.rs every time.

To fix this, separate the inputs to build.rs (the JS files) into a separate directory, bundle/js/, and move the compiled .lrt files to bundle/lrt/. This will keep their timestamps separate.

Checklist

• [ ] Created unit tests in tests/unit and/or in Rust for my feature if needed
• [ ] Ran make fix to format JS and apply Clippy auto fixes
• [ ] Made sure my code didn't add any additional warnings: make check
• [ ] Updated documentation if needed (API.md/README.md/Other)

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[richarddavison]

@refi64 CI is failing because it requires som files in bundle/js to generate compression dict, see: https://github.com/awslabs/llrt/blob/80daaadabeb314e06ebff0574da562c362e08e2a/.github/workflows/ci.yml#L31C10-L33C15 This feels a bit hacky, but I don't want project to build if the required js files haven't been provided.

[refi64]

@richarddavison ah thanks, was just trying to figure out why I couldn't repro it on my end :sweat_smile: looks like that's the only use of bundle/ in the CI scripts so I think it should be working now.