formula.js: consider switching disp-formula output to div

[pkra]

From #413 (comment)

So far, formulas were turned into spans but our new "text formulas" should be divs (since they can contain paragraphs).
Perhaps every disp-formula should be a div.

[pkra]

This "gem" would also no longer be necessary

https://github.com/AmerMathSoc/ams-xml-to-html/blob/9513332438e87c655fe76c1c4a8b9b6d94469573/lib/elements/fn.js#L21-L22

[pkra]

Note: texml frequently puts disp-formula inside p so we'd have to deal with that (e.g., by improving hacks.js)

[pkra]

We have plenty of disp-formula's in p's so this really depends on upstream.

A quick random test shows that most publications have disp-formula's in p's. Still, they seem somewhat infrequent (<10% of disp-formulas) and they almost always seem to appear inside def-list, so maybe they are not always intentional (cf. AmerMathSoc/texml#104)

Either way, it's a philosophical question - are disp-formulas part of a paragraph or not.