fix(gatsby-plugin-mdx): fix filter by heading depth

[trevorblades]

This branch fixes an issue where heading fields on an MDX node can't be filtered by depth. Here's the query that I was using:

{
  allMdx {
    nodes {
      headings(depth: h2) {
        depth
      }
    }
  }
}

In this case, all headings would be returned, regardless of their depth. This was happening because in the headings resolver, the depth arg was defined as a HeadingMdx here, but filtering would only happen if depth is a number.

I changed the logic to only perform filtering when depth is one of the values that HeadingMdx allows, and adjusted the filtering to make this strategy work properly.

[ChristopherBiscardi]

😅 thanks for catching this :)

[trevorblades]

@ChristopherBiscardi the www build failed, but do you think we can merge this in and publish a new version anyway?

[ChristopherBiscardi]

@trevorblades unfortunately merging is tied to WWW passing now. @gatsbyjs/core any idea what the github token issue is here?

[DSchau]

Published in gatsby-plugin-mdx@1.0.23

Thanks @trevorblades!