add IDs to more `<dfn>`s

[michaelficarra]

Most, if not all, <dfn>s would benefit from being able to browse references using the references pane. Unfortunately, the references pane is only enabled for <dfn>s that are given an id attribute, which many currently lack. We should go through and add IDs to any that are reasonable.

[bakkot]

Or possibly all of them.

I think the main harm is in links only going the containing clause, where in many cases the definition only occurs several sentences or paragraphs in to the clause.

tc39/ecmarkup#555