A plain scalar continuation that ends at a `:` is normally a new implicit
mapping key, so the scalar stops before it. Under an explicit key (`? foo`
then `bar :`) the `:` is the key's own value indicator instead, so the line is
still key content. The scalar dropped that line and the parser stalled, so
`? foo` then `bar : baz` failed rather than yielding the key `foo bar`.
When the explicit-key marker is pending, fold the continuation line in and
stop, leaving the `:` to be scanned as the value indicator. An implicit key
keeps the previous behaviour, so a continuation ending at `:` still begins a
new key.
The test-suite rates are unchanged, since no comparable case isolates the
block form and the flow form needs separate explicit-key work, but block
explicit keys spanning lines now fold correctly. 466 tests pass.
