feat: detect bun as package manager from bun.lock file

[anurag-roy]

Description

This PR adds a feature to detect bun as the package manager if bun.lock file is present

Linked Issues

Solves #37

[anurag-roy]

Could anyone please help me with the tests?

I think there should be a test case where we check if bun is being inferred as a package manager when bun.lock file is present, just like we have a test case for bun.lockb.

But the way tests are setup currently, there's a folder for bun inside test/fixtures/lockfile which gets tested by traversing all the directories inside lockfile. So, I cannot just add a bun.lock file inside the bun folder. Should I create a new folder called bun-lock and pass it on here like

const agents = [...AGENTS, 'bun-lock', 'unknown']

[antfu]

To me I am fine with having this without tests as this change should be very straightforward

[anurag-roy]

Can this be merged? @benmccann