The global `scriptFS` variable was too easy to access, which caused an
issue where the mandatory `"scripts"` subdirectory was not passed.
Accessing via a getter function that hides this requirement prevents this.
Take some functions out of the `Service` struct, as they are more or less
standalone anyway. This will also make it easier later to make things
thread-safe, as that'll become important when files can get live-reloaded.
Refactor the JS script file loading code so that it's tied to the `fs.FS`
interface for longer, and less to the specifics of our `embed.FS` instance.
This should make it possible to use other filesystems, like a real on-disk
one, to load scripts.
