Support pre-filled variables in front-matter

[sgrove]

Looking at https://www.onegraph.com/schema/example/GitHubMergePullRequest, it has variables hard-coded in the description. They're not formatted great, and they don't transfer to the try in GraphiQL link.

If graphql-codex supports (ideally commented) json variables in the frontmatter, it can render them nicely and include them in the try it links!