feat: add LaTeX3 matchup support.

[RadioNoiseE]

Added matchup support for a bunch of LaTeX3 macros. (They are documented in source3.pdf and are the stable ones. In l3candidates I didn’t find any experimental macros which need matchup support anyway.)

And here’s a test file which may be helpful: test-expl3-matchup.txt (maybe :setf tex is needed). And there’s something wrong with it: \if_true|false: has no signature w. (just aware of that

N.B., the only changed line is:

" let l:match_words .= ',\\if\%(\w\|@\)*\>:\\else\>:\\fi\>'
let l:match_words .= ',\\if\%(\:w\|\%(\w\|@\)*\)\>:\\else\:\?\>:\\fi\:\?\>'

since the LaTeX3 version (\if:w\else:\fi:) overlaps with the TeX version, and therefore the \: is placed before the \>. Thus a set iskeyword+=\: is needed for correct rendering. And that’s the reason why I didn’t use \> for the other \else:\fi:s.
Is it better for me to append \> after or not?

[andymass]

Hi, I see it was closed but it looks good to me, so merging.

Is it better for me to append > after or not?

If I understand right, it makes sense the way you have changed it.

Thank you for contributing!