Add a writeBundle or buildEnd hook

[JonasKruckenberg]

Is your feature request related to a problem? Please describe.
A good number of rollup plugins use the writeBundle hook, to run tools after all chunks and assets have been emitted, this hook however is never called with the vite build system since instead of using rollups bundle.write method it uses its own.

Describe the solution you'd like
My suggestion would be to either call the writeBundle hook on all rollup plugins manually or implement a custom hook in the vite plugin interface that gets called after all assets have been written to disk.
Describe alternatives you've considered
There is not really a workaround with rollup, you could try to detect filesystem changes manually or something I guess...
You can run any script after vite build has finished but that is really awkward, requiring placeholder scripts in the public folder, I think we can do better than that.

Additional context
Would solve #853 as then rollup-plugin-workbox would work again.

It really should not be a big issue, if you want I can create a pull request for it!

[dominikg]

rollup docs for writeBundle start with "If it is called". So it is not mandatory to call it. rollup-plugin-workbox could try to use generatebundle instead.

I'm not sure you should fake writebundle unless what vite is writing exactly matches what the hook api expects it too. Otherwise you get lots of compatibility issues