Right now (Unicode 15.1), all valid variation sequences consist of a
single Unicode codepoint (followed by either VS-15 or VS-16).
Don't assume this is the case.
We don't actually handle longer sequences. But now we at least catch
such escapes, and error out.
Lookups in this table is not performance critical at all.
Thus, let's compact it to bring down the binary size of foot.
This brings the size of each entry down from 9 bytes to 6, bringing
the size of the whole thing down from 1647 bytes to 1098 bytes, saving
us 549 bytes.
At compile time, build a lookup table from the Unicode data file
'emoji-variation-sequences.txt'.
At run-time, when we detect a VS-15/16 sequence, do a lookup in this
table, and enforce the variation selector iff the sequence is valid.
Closes#1742
