fix(windows): SMP-aware deletion in TSF-aware apps

Fixes #4196.

When deleting characters in a TSF-aware app, we must delete both halves
of a surrogate pair. This behaviour differs from legacy apps, where a
single backspace is usually sufficient to delete both characters.

Truly ancient apps that do not know about Unicode surrogate pairs are
not going to delete both halves with a single backspace event.
Fortunately, these are few and far between; we would handle them on a
case-by-case basis if support questions for them arise.

When we come to integrating Keyman Core into Keyman for Windows, there
will need to be some careful checking of surrogate pair support, as it
is likely that the handling will need to change.