TextInputWidget: Add getValidity function
TextInputWidget: Add getValidity function

This method returns a promise that resolves if the field is valid and
rejects if not. Helpful when you need to check the validity of multiple
fields with a single $.when call.

Bug: T99831
Bug: T107998
Change-Id: I9a027033cb4253b211a91ecb7baf22046bcb7213

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui