Issue #3079

So that the helper mechanism can meaningfully be used to register
asynchronous linters.

Closes #3078

Issue #2064
Issue #3074

Issue #3067

Teach it about @document in a more general way than my first attempt,
and also do @supports the same, by treating both them and @media as
atBlocks with shared word parsing

Issue #3066

Add support for the @supports rule

Issue #3064

Issue #3063

Use host property, not root.

Easiest to see the difference on touch-enabled device. Go to Tern demo
page (link below), click on a variable name and press Ctrl+I. Now pan
the editor's content. The tooltip stays at a fixed position, whilst
the variable name detaches and moves off.

The fix listens to scroll and blur, switching the tooltip off.

Issue #3055

Issue #3039

Issue #3047

Issue #3053

Opening the documentation in the same window navigates away from the
code editor.