Allow partial override of rollup manualChunks

[NuckChorris]

Clear and concise description of the problem

As a developer using Vite, I want to be able to override rollupOptions.manualChunks without losing the default behaviors from Vite, so that I don't have to reimplement the wonderful things Vite does for me

Suggested solution

Two solutions come to mind:

• Exposing the default rollupOptions.manualChunks implementation so that my function can call it
• Special-casing the rollupOptions.manualChunks option and "merging" it with the Vite default (ie, call myChunks ?? viteChunks since the function should already return undefined to use rollup defaults)

Honestly I think either would work nicely!

Alternative

No response

Additional context

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[patak-dev]

@NuckChorris please check #6534, we are going to release it as part of vite 2.9 soon.