chore: upgrade rollup

[benmccann]

This just upgrades to Rollup 2 for now. I was having more trouble with Rollup 3 and think that might be easier after we adopt "type": "module". This does upgrade all the plugins fully though, so it's just the core library that will be left

[vercel]

@benmccann is attempting to deploy a commit to the Svelte Team on Vercel.

A member of the Team first needs to authorize it.

[dummdidumm]

Managed to get it working for Rollup 3. This unvailed some weird conventions I wasn't aware of before, which I replaced with another slightly less weird convention. TLDR there needs to be a way for the different bundles (store/internal/etc) to reference each other without bundling the imported other file (which will be a bundle). Previously it relied on svelte/x and transforming to a relative import, but that was mentioned nowhere and might lead to accidentally wrong packages if someone in the future does a relative import (because that's the more sensible way to import things). Now relative imports are used and the rollup was adjusted accordingly.

[benmccann]

wow. thank you @dummdidumm! that was quite the tricky upgrade