After resolving an annotation's selectors to a `Range`, convert the
range to a `TextRange` object which is stored with the anchor and later resolved
to a concrete `Range` just before inserting highlights. Representing the
anchored region using a `TextRange` allows for the correct text to be
highlighted later if the region has been modified as a result of
inserting other highlights, providing that the text content of the
region has not been changed.

Fixes https://github.com/hypothesis/h/issues/3278
Fixes https://github.com/hypothesis/client/issues/2326
Fixes https://github.com/hypothesis/h/issues/5997