Add YAML to load_data() supported types

I already have a patch for this, since I needed it for my own use. The patch comes with all the same tests as functionality as load_json(). It’s actually trivial; mostly a duplicate of the load_json() function, and since serde_yaml is already in the binary due to it being included to parse the frontmatter, it’s impact on binary size is minimal.

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That should be fine to add.