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.

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

Issue #3974

Even if we don't use promise internally, check for the existence of
`.then` attributes on the return results of a hinting function, and if
so assume it is a promise.

Internally also convert all hinters to use the asynchronous interface
for consistency and simplification of code paths.

Closes #3971

Closes #3972

5.13.4 was a trivial release restoring a LICENSE file.

Issue #3938