And use it in the merge addon to get the proper offsets for merge buttons

Issue #4364

Issue #4288

The kludge of pulling files from github no longer works now
that we have a build process, and since most people have their
own build pipeline now, I don't feel that supporting on-website
builds is that important anymore.

Closes #4166

See https://discuss.codemirror.net/t/addoverlay-removeoverlay-and-opaque/835

Closes #4122