Introduce grapheme clusters

[antonkomarev]

Grapheme clusters — a sequence of one or more code points which are displayed as a single graphical unit. For example, the Spanish letter ñ is a grapheme cluster that contains two code points: U+006E = n ("latin small letter N") + U+0303 = ◌̃ ("combining tilde").

This will be required to represent emojis as well.