Add `value_type` and `mask_style` properties to mask evaluated values using `maskValue`.

• An operation that omits `value_type` continues to work as it does today, assuming data meant for `maskContent`.

• An operation that includes `value_type` should assume XPath evaluation to a value, instead of a node set, implies use of `maskValue`.

• An operation that includes `value_type` should assume XPath evaluation to a node set implies continued use of `maskContent`.

• Use of `mask_style` without `value_type` is an error.