Create development path without compatibility projects

[PureWeen]

Description of Change

The entire compatibility folder is currently not used for any active MAUI development. It currently just slows down the solution load and creates more cognitive load for users.

• I created a new solution Microsoft.Maui-dev.sln. The main reason for doing this is to conditionally include the compatibility project references inside any of our sample's projects. This also lets you load Microsoft.Maui.sln as if the Compatibility project was still present which should also keep CI consistent. I would have done this with a solution filter but it's currently not possible to know if you are running a solution filter or not.
• Created a new compatibility unit test project for the unit tests we had in Controls.UnitTests that dependend on Compatibility.

Other thoughts

• Rename Microsoft.Maui.sln to Microsoft.Maui-full.sln and this is what we build on CI. The Microsoft.Maui-dev.sln in this PR becomes Microsoft.Maui.sln
• Move all of the Compatibility bits into its own Microsoft.Maui.Compatibility.sln and then we just make sure to build this on CI to validate everything compiles.

Work to do

• once this approach is approved, I'll update the applicable docs as well

[PureWeen]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[mattleibow]

Almost there!