fix install for yarn@4

[broofa]

yarn@4 doesn't install this repo properly when referenced via a git: URL in package.json. All you get is this:

$ ls node_modules/thememirror/
package.json readme.md

I assume this is because yarn is enforcing the package.json#files constraints, but also not running the prepare script. 🤷

I played around with adding a postinstall script to this, but this repo uses npm, and trying to get our yarn world to play nicely with that was a bit of a rabbit hole. So... simpler to just build and commit the dist folder here.