Add support for bun

[AndyClausen]

I'd like to deploy an application to Cloud Run from source with bun, but none of the current buildpacks seem to support it. Having the bun runtime available isn't even critical, but at least being able to use bun as a package manager would be a huge step forward so I can use my bun.lock/bun.lockb lockfile.

[jama22]

Interesting. I guess we could use the presence of a bun.lock or bun.lockb file as our detection heuristic for bun. I have no strong objection to adding bun support but right now its just an issue of roadmap and scheduling.

If we get more upvotes for this, we may consider it for future inclusion. PRs would also be welcome