UDFs: add "'range'" converter to replace the "xw.Range" converter

A lot of users still get confused and use `xw.Range` in their scripts while they should be using `mysheet.range`. Removing it in the future from the global namespace will prevent that, but we then need to switch from `@arg('arg', convert=xw.Range)` to `@arg('arg', convert='range')`. Leaving `xw.Range` in there for now though.