Implement base PathObject class with access and mutation methods

Adds new path-based types to ably.d.ts.

The previous LiveObject types are temporarily marked as *Deprecated in
ably.d.ts until they are fully converted to the new type system in the
following PRs.
The `RealtimeObject` entrypoint temporarily adds a `getPathObject` method
to get a PathObject instance for a root and preserves the existing
`.get()` method which returns a LiveMap instance directly so that
corresponding tests do not fail yet.

Resolves PUB-2057

Author: VeskeR