Feat: Implement Scoped Slot string support

[dobromir-hristov]

This PR implements the previously very convenient scoped slot syntax of providing just a template string.

[lmiller1990]

I played with this, seems good. I wonder if we need any tests with shallowMount and the renderDefaultSlot flag?

PS: I fixed the build. We had to specify @vue/compiler-dom as an external dependency the rollup config. This means if you want to use VTU in a browser as an IIFE, you will need to include VueCompilerDOM globally. I think this makes sense.

[lmiller1990]

This is great! Thanks @znck and @dobromir-hristov.

[dobromir-hristov]

OK yeah awesome. As I said I did not have time to spend on it, but glad you figured it out :)))

I think we should make note in the docs that it needs to be present. Jessica's Vite toy project would need it.

Overall I am very very happy with how this turned out. It reduced the complexity by N times, everything is handled by the Vue compiler. Sure we have a hacky string to function thing, but thats fine 😆

I can add more tests if anyone thinks of something? Maybe some not so happy paths?

[lmiller1990]

I have updated the rollup config further here: https://github.com/vuejs/vue-test-utils-next/pulls

We have it listed as a peer dependency, I think this should be okay. For Vite, it will work out of the box, since all the packages in vue-next have the module field. only people using this as an IIFE will need to be aware of this (not sure how common this is, now that es modules work in modern browsers).

Sure, feel free to add more tests - would be happy to merge as is, as well. I'll leave this decision up to you since you own this feature 👍

[znck]

We can't avoid string to function conversion, even full build of Vue uses the same thing:

https://github.com/vuejs/vue-next/blob/b725b63e5fd6b9c9875002c2c23e210293bd771b/packages/vue/src/index.ts#L66-L68