Add placeholder implementation for `Iterator.prototype.join`

To get started on implementing Iterator.prototype.join, we should add a preference for the feature, and use it to control whether or not the new join method is available. Once that's ready, we can do the implementation itself.

First, create a shell option and preference. Please use the name javascript.options.experimental.iterator_join. There's a recent example of how to do this in Bug 1997187.

Second, create an empty placeholder method in Iterators.js and implement the logic to control whether or not it is exposed depending upon the value of the preference. There's a recent example of how to do this in Bug 1998512.

https://hg.mozilla.org/mozilla-central/rev/0539b45bfb8b
https://hg.mozilla.org/mozilla-central/rev/5582d4364927