This doesn't make them fully correct, in particular directives sections
(e.g., "%YAML 1.2" directive before a document start marker) are not
handled correctly, but they weren't handled correctly before, either.
It also doesn't recognise or try to do anything about document end
markers (`...`).
This fix does allow scalar documents to be on the same line as the
document start tag which is valid per examples in the YAML spec,
see for example https://yaml.org/spec/1.2.1/#id2760844
(YAML 1.2.1 spec, section 2.3 Scalars)
It also matches the C++ jsonnet output for std.parseYaml("42\n---"),
which is a stream of two documents, a scalar and then an empty document
(where the empty document is interpreted as JSON null)
