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.
When on-disk job compiler scripts are supported, they will be reloaded
often, and it becomes more important to have the access to the map of
loaded job compilers thread-safe.
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.
The add-on code was copy-pasted from other addons and used the GPL v2
license, whereas by accident the LICENSE text file had the GNU "Affero" GPL
license v3 (instead of regular GPL v3).
This is now all streamlined, and all code is licensed as "GPL v3 or later".
Furthermore, the code comments just show a SPDX License Identifier
instead of an entire license block.
Each job compiler script now must define a `compileJob(job)` function,
which will be called by Flamenco when necessary.
This makes it possible to run the script without a job, and get other
exported symbols from it, such as metadata about which settings its job
type needs/exposes.
