feat(gatsby-transformer-remark): Allow remark subplugins to modify graphql types owned by parent plugin

[stefanprobst]

The schema customization APIs allow modifying existing types only by plugins that own that type, or from a user's gatsby-node.js. This is an issue for "subplugins", which should be allowed to modify types owned by the parent plugin, but currently can't. AFAICS gatsby-transformer-remark is the only plugin which has subplugins.

The approach in this PR is to forward gatsby-transformer-remark's createSchemaCustomization API call to its subplugins, so they have actions bound to gatsby-transformer-remark available. This makes it possible for subplugins to modify types owned by gatsby-transformer-remark.

We forward to a createSchemaCustomization function exported by the subplugin, which maybe is a bit weird because the exports object can already be a function (when the subplugin does not use setParserPlugins or mutateSource, which is most of them I think), but it's "just javascript" (i.e. we just stick it on module.exports.createSchemaCustomization).

Also:

• move auxiliary type definitions (for headings, excerpt formats, wordcount) to createTypes, so they are in a central location and have better names that can be targetted by subplugin's createTypes calls.
• removed a confusing comment that seems left over from ancient times (but maybe i misunderstood)

Related: #13972