Narrator on Windows seems to call ExpandToEnclosingUnit to find the
character immediately following the current cursor position. When
expanding a degenerate range with position `input` (used to represent
the cursor position) to a single-character range, the returned range
should start at `input` and end at `input+1`. Previously,
findTextBoundary() would always return the position before
currentPosition when searching backwards by character, so the result of
ExpandToEnclosingUnit would be off-by-one when expanding to the closest
character.
