feat: Support YAML literal blocks for multiline strings

[ivov]

Thanks for the project!

This PR adds opt-in support for YAML literal blocks for multiline strings in snapshot metadata fields like description and expression. Trailing newlines are preserved via chomping indicators (|-, |, |+).

Set INSTA_YAML_BLOCK_STYLE=1 to enable.

Before:

expression: "fn add(a: i32, b: i32) -> i32 {\n    a + b\n}"

After:

expression: |-
  fn add(a: i32, b: i32) -> i32 {
      a + b
  }

I noticed the YAML lib is vendored and assumed extending it was acceptable - please let me know if you'd prefer a different approach.

Closes #782

[max-sixty]

thanks for the contribution

what's the standard for frontmatter — are these allowed?

I agree this makes it more readable. OTOH it adds another option + introduces backward-compat issues, so need to weigh those...

[ivov]

Thanks for the quick reply :)

what's the standard for frontmatter — are these allowed?

Yes! Literal blocks are standard YAML so any compliant parser handles them, and frontmatter is YAML between --- markers, not a separate spec.

I agree this makes it more readable. OTOH it adds another option + introduces backward-compat issues, so need to weigh those...

Added as opt-in so users control the switch. Older insta versions support this because the parser already supports literal blocks - this PR only extends the emitter. But indeed it's a new option to maintain.

[max-sixty]

yes, another option would be "min-version", which we've thought about a bit before... I think the versioning is basically the only reason to have the config here; so in that case we could just adopt the "min-version" config. Or push this to #456

let me leave this open for a bit to see if others have thoughts (including @mitsuhiko )

thank you again