:sparkles: add basic xunit test project

Added a basic xunit unit test project. Some adjustments were required to make this work:

first ensure Android SDKs are installed and up-to-date, otherwise the build won't launch at all
add net7.0 as target framework to the MAUI project
ensure none of the platform specific code is included in the net7.0 framework build by using multi-target notation
ensure the net7.0 build outputs a library (dll)

This should enable unit testing for shared code. Unsure about platform-specific code.

Closes: #2

igotinfected / burser