Support for file level diagnostic

[pascalberger]

Labels can currently be created for a range or location consisting of line and column inside a file. It would be useful to have the possibility to create labels for a whole file.

API could look like this:

report.AddDiagnostic(
	new Diagnostic("Fix formatting")
        .WithLabel(new Label("Example/Files/Program.cs", "Code should not contain trailing whitespace")
            .WithColor(Color.Yellow)));

Label affecting a whole file could look like this: