YAML requires multi-line flow content to be indented past the enclosing
block context (s-flow-line-prefix). libyaml does not enforce this, so
'flow: [a,' / 'b]' with 'b' in column 0, and the equivalent multi-line
flow mapping, wrongly parsed.
Track whether reaching a token crossed a line break, and in flow context
reject a token at or below the parent block indent. A closing ']' / '}'
or a leading ',' may sit at any indentation, so those are exempt (a flow
mapping's closing brace in column 0 stays valid).
Surpasses libyaml on 9C9N, VJP3/00; whole-suite conformance 394/402,
canonical events 308/308.
Mihaela Mihaljevic
eee7079efe