Makes it possible to select multiple custom script directories in Preferences >
File Paths, replacing the single Scripts path option. Each of these directories
supports the regular script directory layout with a startup file (or files?),
add-ons, modules and presets.
When installing an add-on, the script directory can be chosen.
NOTE: Deprecates the `bpy.types.PreferencesFilePaths.script_directory`
property, and replaces `bpy.utils.script_path_pref` with
`bpy.utils.script_paths_pref`.
Pull Request: https://projects.blender.org/blender/blender/pulls/104876
ba25023d22