Fixes a few issues with Windows text input:
- Filters out ASCII control characters
- Filters out lead surrogates, which aren't valid UTF-16 on their own so will cause assertion failures if sent to Flutter
- Adds a bandaid fix for a crash due to mismatches in indexing in the C++ and Dart text models. (A better fix would be to use UTF-16 and add surrogate pair handling to deletion and forward/back; that will be a later PR since it has a larger scope.)
Fixes https://github.com/flutter/flutter/issues/54879