Improve lazy YAML parsing to apply in defined directives only

[yhatt]

The previous lazy YAML parser cannot parse lazy string when mixed strict string wrapped by quotes. This YAML's color property is always null even if lazyYAML is true.

header: "header"
color: #369

The same case also occurs if there was a not defined directive in Marpit.

color: #f00
notSupported: key

This PR will improve this behavior to allow lazy string only in defined directives.
The new parser will parse strict YAML always, but passed YAML is preprocessed to wrap lazy string by quotes.

Before	Preprocessed
color: #fff backgroundColor: #369 header: Improved "YAML" parser notDefinedDirective: #def	color: "#fff" backgroundColor: "#369" header: "Improved \"YAML\" parser" notDefinedDirective: #def