Tiny library for determining whether a character (in fact, an UTF-8 16-bit value) is an extending character, i.e. doesn't start a new glyph. The second half of surrogate pairs counts as an extending character as far as this library is concerned.
(ch: string) → bool
Check whether the given character is an extending char.
(string: string, pos: number) → bool
Check whether the character at the given position in the string is an extending char.