Support for skipping nodes

[cernymatej]

Currently, it is possible to skip subtrees of nodes by calling this.skip(). However, there is no TS support for this, and more importantly, it doesn't call the leave() function when skipping — which breaks our ScopeTracker integration.

We should probably override the this context with a custom one, and perhaps provide a ctx object in the enter() function too.

However, upstream support for skipping while still calling leave() needs to be added first:
Rich-Harris/estree-walker#41