The primary motivation for this is to make `generate-emoji-txt.sh` more
useful for generating a compact list of new emoji being added (e.g.
for use in commit messages / PRs) if it's run with an emoji image
directory that contains only the new emojis.
Instead of manually updating emoji.txt whenever new emoji are added,
we use Unicode's emoji-test.txt to generate emoji.txt on each build,
including only the emojis that Serenity supports at that time.
By using emoji-test.txt, we can also include all forms of each emoji
(fully-qualified, minimally-qualified, and unqualified) which can be
helpful when double-checking how certain forms are handled.