Unicode v12 was released today:
https://unicode.org/versions/Unicode12.0.0/

This patch updates the following sequence property escapes per
Unicode 12:

- Emoji_Flag_Sequence (no changes)
- Emoji_Keycap_Sequence (no changes)
- Emoji_Tag_Sequence (no changes)
- Emoji_ZWJ_Sequence

Note that Emoji_Modifier_Sequence is still using Unicode 11 data. This
will be fixed automatically once a version of ICU with Unicode 12 support
rolls into V8, at which point I'll update the relevant tests.

Bug: v8:7467
Change-Id: Iba575a471382ba7b029da06c2868c368ff43c649
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1503764Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60052}