Account for isValid() becoming async

TextInputWidget#isValid was added as an async function in OOUI,
but we already had a synchronous isValid() in LinkTargetInputWidget.

Call this function using its new async pattern in LinkInspector,
and get rid of the override in LinkTargetInputWidget in favor
of passing a regex in the constructor.

Bug: 71237
Change-Id: I6cff96aaa64f95114c612e43cce3556cb8c82145
Merge "Account for isValid() becoming async"