Support PowerShell

[superatomic]

Add support for PowerShell (suggested by @Valente-math; thank you!).

PowerShell is different from other shells. A few notes:

• Evaling xshe's output (based on how Starship handles this):

  Invoke-Expression (&xshe powershell)

• PowerShell's documentation on environment variables.
• PowerShell's documentation on quoting rules.

Additionally, adding support for PowerShell means being more serious about supporting Windows. While precompiled executables of xshe for Windows have been available for a while now, there has been no testing for Windows compatibility. The biggest concerns are whether get_file_path_default() will work correctly on Windows, and whether the XDG config directory (default: ~/.config) is the correct default location for xshe.toml on Windows machines. As PowerShell is cross-platform, this is not directly relevant to this issue, but still very important to look into.