wip: Introduces TextController.asMutableTextFieldValueState.

[rjrjr]

• Waiting for the real author of this to get back from pat leave so we can debate replacing this:

  selectionStart: Int = textValue.length,
  selectionEnd: Int = selectionStart,

with this:

  selection: TextRange = TextRange(textValue.length)

• Also probably should punch up the related UI test

[rjrjr]

Closed in favor of #1299