Support user-level installation on Windows

Is your feature request related to a problem? Please describe. User-level installation of the Windows package is not available. For example, winget shows that no user-level installation is configured:

> winget install --scope user StephanDilly.gitui
No applicable installer found; see logs for more details.

The .msi installer in the releases page requires admin access.

Describe the solution you'd like Add the ability to install the program for just the current user i.e. in ~\AppData.