If there is a `scripts` directory next to the current executable, load
scripts from that directory as well.
It is still required to restart the Manager in order to pick up changes
to those scripts (including new/removed files), PLUS a refresh in the
add-on.
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.
