Previously, it was possible for an up/down arrow press to place the
cursor in the middle of a multi-code point grapheme cluster. We want to
prevent this in a way that matches the behavior of other browsers.
Both Chrome and Firefox will map the starting position to a visually
equivalent position in the target line with harfbuzz and ICU segmenters.
The need for this is explained in a code comment. The result is a much
more natural feeling of text navigation.
