Comment on multiline map key is moved to value

[d-enk]

Description

When formatting YAML that uses a multiline block scalar as a map key, comments attached to the key are not preserved.

---

Reproduction

Input:

? | # comment
  multi
  line
  key
: value

After yamlfmt:

? |
  multi
  line
  key
: value # comment

The comment originally associated with the multiline key (? |) is moved to the value line.

---

Expected behavior

The comment should remain attached to the multiline key, or the formatter should report that this construct is unsupported instead of silently moving the comment.

[d-enk]

yaml/go-yaml#203

[braydonk]

I estimate that this would require a pretty major upheaval of the parser and AST node type, so it's unlikely I will ever fix this.