Change dafault version command

[NilayGanvit]

Is your feature request related to a problem? Please describe.
The system default Python version cannot be changed easily when multiple Python versions are installed and managed through pymanager. This causes confusion and errors when running scripts or tools that rely on python pointing to a specific version, especially in environments where different projects require different Python versions.

Describe the solution you’d like
Add a supported and documented way in pymanager to set or switch the default Python version globally and optionally per project. This could be done via a simple command such as setting a global default that updates the python and python3 shims or PATH resolution accordingly.

Describe alternatives you’ve considered

• Manually modifying PATH environment variables
• Relying on virtual environments only, which does not solve the global default issue

These approaches are either error prone, platform dependent, or unintuitive for users already relying on pymanager.

Additional context
This feature would improve developer experience, reduce setup time, and align pymanager with other language version managers that support explicit default version selection.