Update activate.nu to support Linux and MacOS

[tobiasborregaard]

Thanks for contributing, make sure you address all the checklists (for details on how see development documentation)

• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

Changes Introduced

1. Cross-Platform Compatibility:

   • Added detection of Windows, Linux, and macOS using $nu.os-info.name.
   • Dynamically adjusts environment settings based on OS (e.g., binary folder, path separators).

2. Improved Path Handling:

   • Dynamic detection and handling of PATH for Windows (Path) vs Unix (PATH).
   • Ensured compatibility by adding correct path separators (; for Windows, : for Linux/macOS).

3. Prompt Handling:

   • Improved prompt management to reflect the active virtual environment.
   • Preserved previous prompt states for better user experience.

[gaborbernat]

Changelog please.

Please also describe what changes it introduces exactly becuase we already support those platforms (see CI run and test suite).