Support for code splitting

[jakepusateri]

With --experimentalCodeSplitting, rollup will output multiple files when you use dynamic import. Currently, the code splitting target has to be a separate package or it'll just get inlined.

Can that flag be enabled, or should it wait until rollup 1.0 lands?

I tested this locally and it does correctly create multiple files within a package. I'm not sure how it'll work in commonjs-land, but rollup 1.0 makes code splitting on by default so it'll have to work if we want to upgrade to that.

[evocateur]

I can see that as an optional config in the rollup pkg prop. Not sure it's really helpful for building npm package artifacts, however.

[evocateur]

https://github.com/evocateur/pectin/compare/esm-code-splitting has my progress so far. Got it working locally, need to fix the tests, maybe noodle a bit more on the acceptability of always enabling it...