Better handle dead caret / selection text ranges in Microsoft Edge. (#7997)
Specifically:
* No longer support remembering the last caret position for previously visited documents in Edge. This never worked and only caused errors. The UIA textRange we stored could not be used on new documents.
* Detect and fix the case where the textRange we have stored as the browseMode selection dies due to major page mutations. Replace it with the first position in the document in this case.