LibLine: Use grapheme clusters for cursor management

This makes using the line editor much nicer when multi-code-point
graphemes are present in the input (e.g. flag emojis, or some cjk
glyphs), and avoids messing up the buffer when deleting text, or
cursoring around.

Userland/Libraries/LibLine/StringMetrics.h

@@ -27,6 +27,7 @@ struct StringMetrics {
     };
 
     Vector<LineMetrics> line_metrics;
+    Vector<size_t> grapheme_breaks {};
     size_t total_length { 0 };
     size_t max_line_length { 0 };