Update existing manifest?

[Rich-Harris]

Hey — wondering if you'd be interested in a PR that changed the behaviour of options.manifest such that it updates an existing manifest rather than creating a new one each time (unless it doesn't already exist, of course)? My use case is that I have two Rollup configs and also need to hash the filename of some generated CSS — it would be cool if I could put all three in the same manifest.

[JohanBjoerklund]

We have a similar use case where we use the same Rollup config to generate different bundles depending on a flag pointing to different entry files. As it is now, we have to generate two different assets files for those bundles.

[phamann]

@Rich-Harris Apologies for the delayed response here. Yes, very happy for a PR for this change, or detail what needs to be done and i'll update.