feat: add support for bun

[MattMattV]

Hello ! :wave:

I took the liberty to add support for bun, a recent and promising JavaScript tool and runtime.

It features self update, so I was able to derive existing work in the repository.

One thing I was not sure about is in which cfg part in config.rs I should add my step since Bun is only available for Linux and macOS at this time. If I put it in the wrong part, please indicate me, I will move i tto the right place.

Standards checklist:

• [x] The PR title is descriptive.
• [x] The code compiles (cargo build)
• [x] The code passes rustfmt (cargo fmt)
• [x] The code passes clippy (cargo clippy)
• [x] The code passes tests (cargo test)
• [x] Optional: I have tested the code myself
  • [x] I also tested that Topgrade skips the step where needed

[s34m]

Merged at new fork because of inactivity by maintainer.