Support .NET 10 single file projectless compile

[adegeo]

Added detection for a single C# file in a directory with no other code files or projects above or below it. When this is triggered, it uses the C# file name as the project name, which lets us run dotnet build file.cs.

If the host is running .NET 9 or below, this just results in a compiler error because file.cs isn't a project, as expected. If the host is running .NET 10 or above, the compile runs normally and the only error reported is a true compiler error.

I added two additional tests (note that tests don't compile and check that portion of the project, they only test the detection engine):

• Edit - Single file no project - Compile which tests the new .NET 10 projectless compile.
• Delete - Single file find project C# which tests deleting a single file from a directory with a project and some other code file. This doesn't test anything new, it was just previously omitted.

[BillWagner]

This lgtm @adegeo lets