Closes #4025

Fix "Uncaught TypeError: Cannot read property 'severity' of undefined"

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

Issue #3941

Closes #3934

Closes #3920

Closes #3891

Closes #3802

Closes #3849

When hovering a line with multiple lint annotations, codeMirror will
now show all the attached markers instead of just the first one

Closes #3847

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

Closes #3802

When enabled, and the annotatescrollbar addon is loaded, matches will
be shown on the scrollbar.

Since the viewport info isn't updated until the redraw, and the event
fires before the redraw.

See https://discuss.codemirror.net/t/whats-wrong-fold-gutters-do-not-show-up-after-swapdoc-call/580

Issue #3733

To avoid IE8 parser issue.

Issue #3636
Issue #3698

Closes #3703

So that tiny mouse motions don't keep resetting the selected item

Closes #3698

Issue #3682

Closes #3651

Make this the default

Issue #3636

Moving the mouse will change the selected hint option

So that the annotations still make sense when scrollPastEnd is enabled

Closes #3630