refactor(workshop): edit Barotrauma config_player.xml with sed instead of Python

[dgibbs64]

Description

Replaces the inline Python in fn_workshop_bt_packages (from #5014) with sed, keeping the module pure bash. Behaviour is unchanged.

GNU sed -z reads the file as one record, so the patterns match Barotrauma's <package> elements, which span two lines. The steps are:

1. Expand <regularpackages />.
2. Remove LinuxGSM's entries for removed and current Workshop mods.
3. Add the installed mods, in workshopmods order, before </regularpackages>.
4. Collapse an empty element back to <regularpackages />.

Entries you added yourself are untouched. Unlike the Python version, the file's UTF-8 BOM is now kept.

Testing

• Live test server (btserver):
  • Installing 3815618949 added its <package path="LocalMods/3815618949/filelist.xml" /> entry.
  • Removal deleted it and put back <regularpackages />, so the file matches the original.
  • The server stayed healthy throughout and was restored afterwards.
• Locally, the same scenarios as feat(workshop): add Steam Workshop downloads for Barotrauma and Stationeers #5014 give identical output:
  • The self-closing element is expanded, and the result stays valid XML.
  • Order follows workshopmods.
  • A hand-added entry and its comment are kept.
  • Removed mods' entries are dropped.
  • The game's two-line entries for other mods are untouched.
  • The BOM is kept.
• shellcheck and prettier pass.

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.

Relates to #3514.

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