fix(gatsby-plugin-mdx): set childOf extension for Mdx type definition

[tesseralis]

Description

Add the childOf extension to the Mdx node type to aid in populating types.

This is equivalent to:

type Mdx implements Node @childOf(mimeTypes: ["text/markdown", "text/x-markdown"]) {
  # (or whatever is defined in `options.mediaTypes`
}

Motivation

This makes the typing more robust so that anything with @mimeTypes(types: ["text/markdown", "text/x-markdown"]) immediately get childMdx even if no nodes are created.

[vladar]

Looks good to me!