Generate skin tone compositions from emoji-sequences.txt

Generate skin tone compositions from emoji-sequences.txt

Read skin tone modifier sequences from emoji-sequences.txt and add
them to the per-character composition rules derived from
emoji-zwj-sequences.txt, rather than adding them as lookback rules for
the skin tone modifiers.  This avoids an issue with the application of
such lookback rules.  See Bug#39799,
specifically
<https://lists.gnu.org/archive/html/bug-gnu-emacs/2021-09/msg01878.html>
for more details.

* admin/unidata/Makefile.in (zwj): Add emoji-sequences.txt as a dependency.
* admin/unidata/emoji-zwj.awk: Match RGI_Emoji_ZWJ_Sequence and
RGI_Emoji_Modifier_Sequence.  Remove manual addition of skin tone
composition rules with lookback.