fix(service): fixes memory issue, adds math processing behind flag

This MR does two things:

1. This implements the exec/`spawn` fix suggested in #4 in `DOCXToHTMLSyncHandler`, which makes one of our problematic documents pass through correctly.

2. Behind a flag, I've put in some math fixes. If (and only if) there's a useMath in the form-data, I run a function on <math-display> and <math-inline> that reduces the number of escaped backslashes. Here's what you see (in Postman) without this:

This is what the service does now. I've highlighted a <math-display> element; in it, you can see that the LaTeX has 4 backslashes (`\\\\frac`). If I check the useMath setting, here's what I get:

There are 2 backslashes (`\\frac`) which is better – you can see that JSON also escapes quotation marks in <p class="paragraph">, and when these are unescaped we should end up with what we want, 1 backslash (`\frac`).

I don't know if this is a wonderful fix, though it will solve some of the problems we've been seeing. LaTeX embedded in HTML is inherently problematic – there are < and > and \ and & which easily get busted, and JSON and HTML's escaping aren't helping (to say nothing of whatever's happening with the command-line script). It might be smarter to turn it into base-64 to send it back?