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

`serializeable` → `serializable`
Replace `an` with `a` 
Remove extra `command`

This patch enables styling of active line numbers by
adding a `CodeMirror-activeline-gutter` class to the
line's gutter space.