[Diff mode] Allow clicking on variables that are in modified text

[caraitto]

1. Modify a spec, build the diff. For instance, click on the "Diff" link in the "Preview | Diff" section of Spec: Fix error and not requested cases around trusted bidding signals WICG/turtledove#1180. (I'm not sure how to do this from the command line, it doesn't seem to be documented -- maybe it's another tool and not Bikeshed that does this, or maybe not? I couldn't figure it out looking at the GitHub build log for that PR.)

2. Find a yellow section of modified text. You can use the "j" and "k" keys to quickly go to next and previous diffs.

3. Click on a variable referenced in the yellow text.

EXPECTED: All instances of that variable get highlighted, just like would happen for non-modified text.

RESULT: Nothing happens. However, if that variable is referenced somewhere outside the diff'd region, clicking on that instance will highlight all occurrences, including those in the diff'd region.