Add more emoji sequence properties

[mathiasbynens]

• Emoji_Combining_Sequence
• Emoji_Flag_Sequence
• Emoji_Keycap_Sequence
• Emoji_Modifier_Sequence
• Emoji_Tag_Sequence
• Emoji_ZWJ_Sequence

[mathiasbynens]

Emoji_Combining_Sequence support is not particularly urgent, as it might be removed: https://unicode.org/draft/reports/tr51/tr51.html