Make Python runner cross-platform

[defcatcher]

This PR updates the Python execution flow so the IDE can run Python scripts on Windows, macOS, and Linux.

What changed

• Added platform-aware interpreter fallback for installed Python
• python3 is tried on macOS/Linux before python
• Windows keeps support for python, python3, and py
• Prevents spawn py ENOENT crashes on non-Windows systems
• Made the Python runner setting visible outside Windows as well

Notes

• The built-in Python runtime remains Windows-only for now
• On macOS/Linux, the IDE will use the system Python installation

[cdmtn-dev]

Looks good! Clean refactor to async/await and cross-platform support.

A few quick notes:

• No tests — This is critical path code. Consider adding unit tests for the platform detection and fallback logic.
• Manual testing needed — Verify on Windows (embedded + fallback) and macOS/Linux before merge.
• Error visibility — The fallback loop silently catches all errors; consider returning what was tried for debugging.

Otherwise, solid changes. Request a maintainer review before merging.