[Binding SG] Split bindings source generated code into multiple files

[jkurdek]

Description of Change

This is a follow-up to #21725, contributes to #22384

This change generates each SetBinding in a separate file allowing for more granular caching.

FIXES

#23551

[jonathanpeppers]

FYI the changes here were part of:

• Binding source generator #21725

But we lost them by accident during a rebase.

[simonrozsival]

There was a problem with the Essentials.Sample.Server.WebAuthenticator.csproj sample project - the Binding Source Generator was loaded from the .buildtasks directory, and it produced code even though the project didn't reference MAUI at all. I cannot reproduce this with the same project when it is outside of the MAUI repository.

I am not sure if this is due to a presence of Directory.Build.targets or some other implicit behavior of MSBuild, but I don't think this is an issue outside of the MAUI repository. I suppressed the analyzers with an MSBuild property in that particular project for now.

Alternatively, we can change the SG to only generate the common code if the project actually references Microsoft.Maui.Controls.dll. Feedback is welcome.