A plain scalar continuation stops before a line ending at `:`, since a block
implicit key may not span lines and the next line is a new key. A flow key may
span lines, so the same rule wrongly split `{ multi\n line: v }`: the scalar
returned just `multi` and the parser mis-read `line: v` as a separate entry.
Fold across the `:` in flow as well, leaving the `:` to be scanned as the
value indicator, so the key folds to `multi line`. A block implicit key keeps
the previous behaviour and still stops before the `:`.
Raises yaml-test-suite parse-level conformance to 301/333 (90.4%) and
value-level to 193/195, with the should-error rejection rate unchanged.
476 tests pass.
