Validate assembly names passed in the MtouchInterpreter msbuild property

[simonrozsival]

There are no build warnings or build errors when the assembly names in the $(MtouchInterpreter) property are in the wrong format or if there is no such assembly in the project. It is hard for the developer to diagnose the problem. Customers are likely to run into this issue because of a bug in the docs which suggest listing assemblies with the .dll extension when Xamarin compares comma separated items in the property with the assembly file names without an extension.

Steps to Reproduce

1. Create a new iOS app with dotnet new ios -o TestMtouchInterpreter
2. Add <MtouchInterpreter>-all,TestMtouchInterpreter.dll</MtouchInterpreter> based on the documentation (https://learn.microsoft.com/en-us/dotnet/maui/macios/interpreter?view=net-maui-8.0, see Fix mono interpreter examples docs-maui#2141)
3. Build the app in Release mode

Expected Behavior

The TestMtouchInterpreter.dll assembly is not AOTed and instead it is interpreted.

Actual Behavior

The TestMtouchInterpreter.dll assembly is AOTed and not interpreted.

Other examples of invalid inputs

<MtouchInterpreter>-all,+TestMtouchInterpreter.dll</MtouchInterpreter>