Generalize methods returning Node type

[falsandtru]

All methods returning the node type value should return their actual type, like #283(cloneNode) and #12048(appendChild).

TypeScript Version: master

Expected behavior:

e.g.

    insertBefore<T extends Node>(newChild: T, refChild: Node | null): T;

Actual behavior:

    insertBefore(newChild: Node, refChild: Node | null): Node;

[RyanCavanaugh]

@mhegazy ...aren't you busy? 😨

[mhegazy]

Just sitting there doing nothing; the wife is doing most of the work :D