Expose a way to inject more rollup plugins during the build phase

[samccone]

Currently if I want to run some custom processing during the build phase I have no way to hook into the hardcoded rollup plugin config. It would be nice to expose a way to add in plugins in as a build escape hatch.

[samccone]

Rich let me know if this violates any design principles you are following. If not happy to propose a change.

[benmccann]

@samccone I'm curious - what's the processing that you want to do?

[samccone]

I would like to do some compile time environment variable replacements in the FE depending on my deployment target (staging|prod) ect.

(to be clear I was able to do this with sapper previously)

[Rich-Harris]

Now that we're using Vite, we can add Rollup plugins to vite.config.js