`[OutputType([xml])]` should be added to XML code paths

[StartAutomating]

Several code paths within the turtle will output XML (or null). These properties should have the output type attribute for clarity and in expectation of future use. Additionally, script properties should typecast to [xml] ([xml]$x), rather than coerce to [xml] ($x -as [xml]). Because they are script properties, they will already hide the casting error from the user, and we do not want the error to be hidden from `$error, as well.