feat: add writeBundle hook

[lforst]

This PR adds the most basic form of the writeBundle hook.

The reason for having it even in that form is that the buildEnd hook is called before any files have been written to disk and the added writeBundle hook could be used to have plugin code run after bundles have been written.

Also adds some tests to verify that the hook has run after assets have been written to disk.

[lforst]

I don't quite understand why the macos job is failing with:

  pnpm run build
  shell: /bin/bash -e {0}

> [email protected] build /Users/runner/work/unplugin/unplugin
> tsup

sh: tsup: command not found
 ELIFECYCLE  Command failed.
 WARN   Local package.json exists, but node_modules missing, did you mean to install?
Error: Process completed with exit code 1.

Maybe someone can help me out here? Ubuntu tests pass.

Edit: Nevermind, was just a flake 🤷

[antfu]

I think it would be better to have a note saying this hook does not pass any arguments.

[lforst]

Good point! I added a note in d1f9fd0.

Btw feel free to make any edits to this PR as you see fit!