Editorial: fix linking and change spelling of iterator interfaces

[michaelficarra]

Fixes #3455. This adds a bunch of <dfn>s and the auto-linking is sooo much better. Also makes a lot of progress getting rid of the remaining <i>s that are used as pseudo-<dfn>s.

edit: Actually, this eliminates all of the remaining <i>s! Nice!

[bakkot]

This also fixes https://github.com/tc39/ecma262/issues/3426