A flow sequence element that begins with `:` is a single-pair mapping with an
empty key, e.g. `[ : value ]` is `[{null: value}]`, but parseFlowSequence ran
the element through parseNode, which found the leading `:` and raised
"expected a YAML node".
Detect the leading mapping-value indicator and emit a single-pair flow mapping
with a null key and the following value (itself possibly empty), the sequence
counterpart of the scalar-key element `[ a: b ]` that already becomes a
mapping through its implicit-key marker. Plain elements and scalar-key mapping
elements are untouched, and the allowance is confined to flow sequence element
position.
Raises yaml-test-suite parse-level to 298/333 (89.5%) with the should-error
rejection rate unchanged. 426 tests pass.
