Adding support for Vue 3's <script setup>

[Jengamon]

I want this to happen, because it would enhance my experience with Parcel and Vue 3.
So I made a pull request here, but maybe I should have done this first.

All I did was call compiler.compileScript with the descriptor block, then use the output to replace the script field. This should mean that none of the other machinery changes.

Also, this is a call for unit tests of these kinds of SFCs. Thanks.

Edit: To get everything working together properly, it did take a little bit more effort, like telling the template compiler what names the script references, or propagating production mode to all 3 compilers to make sure their names for stuff are all aligned.

[Jengamon]

It's been merged yay!