preprocessor: support formatting yaml files

[myitcv]

Per https://github.com/cue-lang/cuelang.org/blob/bcf703c3b2d4cfbbab3d0b5cb779534793d4c223/internal/cmd/preprocessor/cmd/txtar_node.go#L267-L270 this is still not supported.

Points to consider:

• Multi-value yaml files
• The fact that cue export --out yaml drops comments, but going via cue import retains the comments (although runs into cue fmt-related issues)

[myitcv]

~This was fixed in https://review.gerrithub.io/c/cue-lang/cuelang.org/+/1173332~

[myitcv]

This is still an issue. Description updated to be current.