Detect if grapheme is an emoji

This adds the necessary logic to detect if a grapheme cluster is an
emoji based on @rivo's [comment](rivo#27 (comment))

Fixes: rivo#27

grapheme.go

@@ -103,6 +103,11 @@ func (g *Graphemes) Bytes() []byte {
 	return []byte(g.cluster)
 }
 
+// IsEmoji returns true if the current grapheme cluster is an emoji.
+func (g *Graphemes) IsEmoji() bool {
+	return IsGraphemeClusterEmoji([]byte(g.cluster), g.Width())
+}
+
 // Positions returns the interval of the current grapheme cluster as byte
 // positions into the original string. The first returned value "from" indexes
 // the first byte and the second returned value "to" indexes the first byte that
@@ -343,3 +348,63 @@ func FirstGraphemeClusterInString(str string, state int) (cluster, rest string,
 		}
 	}
 }
+
+const (
+	regionalIndicatorA = 0x1F1E6
+	regionalIndicatorZ = 0x1F1FF
+)
+
+// IsGraphemeClusterEmoji returns true if the given byte slice grapheme cluster
+// and width is an emoji according to the Unicode Standard Annex #51, Unicode
+// Emoji.
+func IsGraphemeClusterEmoji(cluster []byte, width int) bool {
+	if width != 2 {
+		return false
+	}
+
+	for i := 0; len(cluster) > 0; i++ {
+		r, rw := utf8.DecodeRune(cluster)
+		if i == 0 {
+			if r >= regionalIndicatorA && r <= regionalIndicatorZ {
+				return true
+			}
+			if propertyGraphemes(r) == prExtendedPictographic &&
+				property(emojiPresentation, r) == prEmojiPresentation {
+				return true
+			}
+		}
+		if r == vs16 {
+			return true
+		}
+		cluster = cluster[rw:]
+	}
+
+	return false
+}
+
+// IsGraphemeClusterInStringEmoji is like [IsGraphemeClusterEmoji] but its input
+// is a string.
+func IsGraphemeClusterInStringEmoji(cluster string, width int) bool {
+	if width != 2 {
+		return false
+	}
+
+	for i := 0; len(cluster) > 0; i++ {
+		r, rw := utf8.DecodeRuneInString(cluster)
+		if i == 0 {
+			if r >= regionalIndicatorA && r <= regionalIndicatorZ {
+				return true
+			}
+			if propertyGraphemes(r) == prExtendedPictographic &&
+				property(emojiPresentation, r) == prEmojiPresentation {
+				return true
+			}
+		}
+		if r == vs16 {
+			return true
+		}
+		cluster = cluster[rw:]
+	}
+
+	return false
+}