This revision implements UIA's TextUnit_Format, a TextUnit used to position the
boundary of a text range based on the formatting attributes of the text. This
revision modifies FindTextAttrsStart such that it does not stop at container
boundaries, and will traverse to the next or previous leaf if necessary. With
this change, FindTextAttrsStart matches FindBoundary in that it returns the true
"start" of the text run. This revision then adds a call to FindTextAttrsStart in
UIA code, and many tests exercising TextUnit_Format.
Differential Revision: https://phabricator.services.mozilla.com/D226853