Closes #4478

Issue #4417

Issue #4413

Adds an option to keep the active line highlighted even when text
inside the line is selected.

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