`citeproc-js` heads-up: change to return value of `hanging-indent`

This is a notice about a small upcoming change to processor output.

In a two-year-old issue filed in the citeproc-js tracker Derek Sifford pointed out that when rendering a bibliography with hanging indent, citeproc-js uniformly returns a hangingindent value of 2; but the CSL Specification states that the value should be true or false.

Zotero code has assumed a numeric value, but since it is always 2 or nil, the value can be hard-coded in the few places where it is used. I’ve filed a PR, but I’m raising a signal here in case there are other citeproc-js consumers that would be hit by a similar issue when the processor is aligned with the spec.