Rmd YAML header?

[gaborcsardi]

This is pretty difficult, unfortunately, but could not come up with a
simpler solution.

• We use tree-sitter to parse and search the YAML, so that we can have
  coordinates, and also the search is much simpler than when using a YAML
  parser.
• The tree-sitter parser cannot scan the actual values of the scalars, so
  we use a YAML parser for that (libyaml). (No, it is not better to scan
  them manually, they are quite involved.)
• Scanning the scalars is a transformation, not just a subsetting, so
  we lose the correct coordinates for the things (e.g. R code) within
  the values. We still have the coordinates for the values, though.
• We don't handle references correctly, because the tree-sitter parser
  does not help with that. For that we'd need to parse the whole YAML
  with libyaml. Maybe we'll do that in the future.