feat(core): add support for positron ide (#15045)

[kapsner]

Summary

Adds support for Positron IDE.

Details

Adds IDE integration of Positron, enabeling IDE-related slash-commands.

Related Issues

closes #15045

How to Validate

• pull this PR
• Install Positron IDE
• open Positron and from integrated terminal install the code from this PR
• run "./bundle/gemini.js" and "/ide install" --> should install gemini-cli-companion extension and output messages such as "Positron companion extension was installed successfully"

Pre-Merge Checklist

• [ ] Updated relevant documentation and README (if needed)
• [x] Added/updated tests (if needed)
• [ ] Noted breaking changes (if any)
• [x] Validated on required platforms/methods:
  • [ ] MacOS
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
    • [ ] Podman
    • [ ] Seatbelt
  • [ ] Windows
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
  • [x] Linux
    • [x] npm run
    • [x] npx
    • [x] Docker