feat(workshop): add Steam Workshop downloads for Squad and Starbound

[dgibbs64]

Description

Extends the Workshop downloader from #5008 to Squad and Starbound. Neither dedicated server downloads Workshop mods itself.

Unlike Arma 3 and DayZ, Squad and Starbound Workshop items download anonymously, so no Steam account is needed beyond what the server already uses (Squad is anonymous; Starbound already needs steamuser).

Per-game install

• Squad: each mod installs to SquadGame/Plugins/Mods/<id>. The server mounts every mod in that folder, so no config change is needed.
• Starbound: each mod installs to serverfiles/workshop/@<id>, and its .pak files are linked into serverfiles/mods as workshop-<id>-contents.pak. Removing the mod removes its links.

Other changes

• Safer clean-up: only folders LinuxGSM installed (those with a .lgsm-workshop state file) are ever removed. This matters for Squad, whose Plugins/Mods also holds the game's own CanadianArmedForces mod and any mods added by hand.
• update, check-update and start now detect Workshop use from the saved mod list (lgsm/data/<instance>-workshop.txt), rather than from serverfiles/workshop. The list is removed once no Workshop mods are left, so update stops checking the Workshop.
• workshopmods was added to the Squad and Starbound default configs, and workshop-update to their commands.

Testing

Live test servers, running this branch:

• Starbound (workshopmods="3816094872", Vanilla Dungeons and Encounters, 27 MB):
  • Downloaded, installed and linked, then the server restarted.
  • starbound_server.log shows Detected asset source named 'nonffs_dungeons' at '../mods/workshop-3816094872-contents.pak' and Loading assets from that file.
• Squad (workshopmods="3813515660", YM NoDrones), downloaded with an anonymous login:
  • The server log shows Successfully mounted and serialised mod: YM_NoDrones from Plugins/Mods/3813515660.
• Removal on both:
  • Clearing workshopmods removed the mod (and the Starbound link) and restarted the server.
  • Squad's CanadianArmedForces was left in place.
  • Both servers stayed healthy throughout and were restored afterwards.
• Locally:
  • A hand-added mod in Squad's Plugins/Mods is never touched.
  • The saved list is removed once no mods are left.
• shellcheck and prettier pass.

Relates to #960 (Starbound Workshop support).

Type of change

• Bug fix (a change which fixes an issue).
• New feature (a change which adds functionality).
• New Server (new server added).
• Refactor (restructures existing code).
• Comment update (typo, spelling, explanation, examples, etc).

Checklist

• This pull request links to an issue.
• This pull request uses the develop branch as its base.
• This pull request subject follows the Conventional Commits standard.
• This code follows the style guidelines of this project.
• I have performed a self-review of my code.
• I have checked that this code is commented where required.
• I have provided a detailed enough description of this PR.
• I have checked if documentation needs updating.

Documentation

Covered in GameServerManagers/LinuxGSM-Docs#132.

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