Allow subobjects in markdown frontmatter arrays to link to files

[KyleAMathews]

This was a tricky issue that made me work at remembering design decisions from
early this year. Like that saying goes, code you wrote 6 months ago might as well
have been written by someone else :-)

Anyways, the problem was their was some YAML data in a markdown frontmatter that looked like:

blocks:
- head: Servers
  image: ../../assets/images/servers.png
- head: Another
  image: ../../assets/images/another.png

So in JavaScript, an array of objects with the image field pointing to files.

Which we'd assume would be inferred as a link to a File node but it wasn't until
this PR fixed things by fixing

• our ability to find the parent node while inferring fields
• setting (new convention) ___PARENT on each subobject in the frontmatter
• and using that to complete the query.

[KyleAMathews]

/cc @fk

[fk]

🎉

[fk]

Thank you Kyle! 🤗

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[KyleAMathews]

Looks like things are working 👍

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