Drop type property from all but Scalar nodes

[eemeli]

Originally, type was a thing that was carried into the AST nodes from the previous CST parser. Now, it's a clumsy & sometimes misleading API. So let's drop it elsewhere, but leave it for scalars where it still makes good sense. This is yet another BREAKING CHANGE.

For scalars, let's move the constants from e.g. Type.BLOCK_LITERAL to Scalar.BLOCK_LITERAL. Otherwise, everything stays pretty much the same.

For collections, we need to add a new flow: boolean property that can now be more easily set or toggled to control whether to use block or flow representation. As before with type: 'FLOW_MAP' | 'FLOW_SEQ', setting flow: true will force that choice for all of its child nodes as well.

Dropping the type values also made it clear that Merge is no longer a good idea as a separate class; dropping it allows for merge nodes to be duck-typed when merge: true or version: '1.1' is set.

-import { Scalar, Type, YAMLMap, YAMLSeq } from 'yaml'
+import { isMap, isSeq, Scalar, YAMLMap, YAMLSeq } from 'yaml'

let scalar: Scalar
-scalar.type = Type.BLOCK_LITERAL
+scalar.type = Scalar.BLOCK_LITERAL

let map: YAMLMap
-map.type = Type.FLOW_MAP
+map.flow = true
-const mapIsMap = map.type === Type.FLOW_MAP || map.type === Type.MAP // only if parsed
+const mapIsMap = isMap(map)

let seq: YAMLSeq
-seq.type = Type.FLOW_SEQ
+seq.flow = true
-const seqIsSeq = seq.type === Type.FLOW_SEQ || seq.type === Type.SEQ // only if parsed
+const seqIsSeq = isSeq(seq)