[TeX input] \dag, \ddag

Inspired by https://github.com/arxiv-vanity/engrafo/pull/168.

We happen to have \dagger and \ddagger (though the packages I found on CTAN were pretty obscure so I'm not sure how that happened).

Since \dag and \ddag are plain LaTeX2e macros and we have similar macros, I feel like we should support them.