Description

This PR updates to yaml / to yml to emit YAML that is closer to YAML 1.2 plain-scalar rules while still preserving string semantics when quoting is required.

The main change is that string values are no longer quoted by default. Instead, the emitter now only quotes scalars when quoting is necessary for correctness. In addition, multiline strings are emitted as YAML literal block scalars with correct chomping behavior.

User-facing changes (Release notes)

More idiomatic YAML output with smarter string quoting

The previous emitter always quoted string values, which produced valid YAML but was more conservative than necessary and less aligned with YAML 1.2 plain-scalar rules.

This change makes output:

• more readable
• closer to idiomatic YAML
• more spec-aligned for plain scalars
• still safe for roundtripping by quoting values that would otherwise be reinterpreted as non-strings

Before:

value: 'off'
path: '/dev/stdout'
listen: '0.0.0.0:8444,0.0.0.0:8445 ssl'
name: 'kong'
kind: 'Deployment'

After:

value: 'off'
path: /dev/stdout
listen: 0.0.0.0:8444,0.0.0.0:8445 ssl
name: kong
kind: Deployment

Multiline strings now emit as:

string: |-
  Hello
  world

Additional notes

Examples from #18010 and #16072 were used for testing and creating new tests.