Tests aren't fully green yet, but I've made one pass for this new approach.

We weren't using this at all

This will allow our CLI to call this from the outside and provide an appropriate --target for each slice.

This keeps the assets per slice completely self contained, and will allow us to have abitrarily deep structures of compiled slice asset directories while still retaining clarity.

This is invalid now that we're targeting single app or slice assets.

This means that .ts files will have .js extensions when referencing them, as before.

Whoops

This means there’s now only one place we need to determine the root: index.ts

These are in two places for now, but at least this makes this special string a little clearer.