These functions are the equivalent of 23b424a46 for grapheme clusters.
In order to add the `is_grapheme_boundary` function we also need to
query whether a byte index lies on a character boundary, so this change
also adds `is_char_boundary`.
This library is a fork of the great library
tui-rs. We've mainly relied on the double
buffer implementation and render diffing, side-stepping its widget and
layouting.