feat(mods): add BepInEx mod for Valheim

[h3o66]

Description

Add BepInEx framework for Valheim

Fixes #3840

Type of change

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

Checklist

PR will not be merged until all steps are complete.

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

Documentation

If documentation does need updating either update it by creating a PR (preferred) or request a documentation update.

• User docs: https://github.com/GameServerManagers/LinuxGSM-Docs
• Dev docs: https://github.com/GameServerManagers/LinuxGSM-Dev-Docs

Thank you for your Pull Request!