Compile within single slice only

[timriley]

Update our esbuild wrapper to compile a single assets directory at a time.

Previously, we crawled the entire app structure to find all assets directories (within the app and slices) and processed them all at once. This had some downsides:

• It made our esbuild wrapper and plugin complicated.
• It meant that we had to take extra steps to prevent naming conflicts for assets with the same name in different slices.
• The above also meant that every slice's assets had to be referred to with the slice name as a leading namespace (e.g. "admin/app.js" for an admin slice), which didn't fit well with the idea of slices being fully self-contained.
• In general, it meant we had redundant functionality in JS code (such as locating slices) that already existed in our Ruby code.

By processing a single assets directory at once, we have a much simpler interface with esbuild: we're not pushing it to do things outside its comfort zone. It also means we can eliminate that redundant logic and continue to leverage our Ruby code as the canonical source of knowledge around slices and the overall Hanami app structure.

To process a single directory and output the compiled files into the relevant location, we expect these to be passed as two new arguments when invoking config/assets.js:

• --path: the path for the app or slice containing the assets to process; e.g. app or slices/admin
• --dest: the directory to putout the compiled files; e.g. public/assets or public/assets/admin (in the case of the admin slice)

These args will not need to be supplied by the user. Instead, they'll be provided by the hanami assets compile and hanami assets watch commands, which will determine the slices with assets and then invoke one config/assets.js process per slice, along with the relevant CLI arguments. This means the user experience is still simple and streamlined.