set EnableMauiXamlDiagnostics to true in templates, and for all debug builds

[StephaneDelcroix]

	public bool EnableDiagnostics
	{
		get
		{
			if (Options.IsTrue("build_metadata.additionalfiles.EnableDiagnostics"))
				return true;
			if (Options.IsFalse("build_metadata.additionalfiles.EnableDiagnostics"))
				return false;
			return Options.IsTrue("build_property.EnableMauiXamlDiagnostics");
		}
	}