Support for Unicode emoji sequences

Support for Unicode emoji sequences

This covers both sequences using Zero-Width-Joiner codepoints and
those without. Bug#39799, I hope.

* .gitignore: Add emoji-zwj.el
* admin/notes/unicode: Add emoji-zwj-sequences.txt and
emoji-sequences.txt references.  Describe how to test after updating
to a newer Unicode version.
* admin/unidata/Makefile.in (all): add emoji-zwj.el as a dependency.
(emoji-zwj.el): Add target plus rules for building.
(gen-clean): Add emoji-zwj.el.
* admin/unidata/README: Add emoji-zwj-sequences.txt and
emoji-sequences.txt references.
* admin/unidata/blocks.awk: Force emoji script to be used for certain
codepoints that are used by the Unicode sequences.
* admin/unidata/emoji-sequences.txt: New file.
* admin/unidata/emoji-zwj-sequences.txt: New file.
* admin/unidata/emoji-zwj.awk: New file.  Derives
composition-function-table rules from emoji-zwj-sequences.txt, plus
hardcodes some derived manually from emoji-sequences.txt.
* etc/NEWS: Announce change.
* lisp/international/characters.el: Load the generated emoji-zwj.el
* src/Makefile.in (emoji-zwj): New target.
(temacs): Add emoji-zwj as a dependency.