Refactor rest of src/ as TypeScript

[eemeli]

In addition to internal changes, there are some external changes as well:

• The base class used by nodes is no longer exported. Internally it's been renamed as NodeBase, and its existence should be considered an implementation detail. If you were using it e.g. for instanceof Node checks, similar functionality is now provided by:

import { isAlias, isCollection, isDocument, isMap, isNode, isPair, isScalar, isSeq } from 'yaml'
isNode(foo) // returns true or false

• The AST and Schema TS namespaces are dropped, as everything now has proper types.
• All output Node types are unions of implementations, with the above is* functions working as type guards for them.

There are two main reasons for switching away from relying on instanceof:

1. Being able to separate the checks to a different internal module than their implementations allowed for all dependency loops to be resolved. It was getting rather hairy otherwise.
2. This should make the library more resilient against dual package hazards.

I am aware that the docs aren't updated here yet; I'll need to get to that separately.