[YAMLParser] Unfold multi-line scalar values (#70898)

Long scalar values can be split into multiple lines to improve
readability. The rules are described in Section 6.5. "Line Folding",
https://yaml.org/spec/1.2.2/#65-line-folding. In addition, for flow
scalar styles, the Spec states that "All leading and trailing white
space characters on each line are excluded from the content",
https://yaml.org/spec/1.2.2/#73-flow-scalar-styles.

The patch implements these unfolding rules for double-quoted,
single-quoted, and plain scalars.