feat(gatsby): DSG config for FS routes

[LekoArts]

Description

Enables DSG in File System Routes via:

export const config = () => {
  return {
     defer: true
  }
}

Documentation

We'll also need to update https://v4.gatsbyjs.com/docs/reference/rendering-options/deferred-static-generation/ and https://v4.gatsbyjs.com/docs/how-to/rendering-options/using-deferred-static-generation/

Related Issues

[ch38465]

[herecydev]

Will this also be the preferred mechanism for adding context into file system routing?

export const config = () => {
  return {
     defer: true,
     context: {
       foo: "bar"
     }
  }
}

[LekoArts]

Eventually, yes. But for this PR it's out of scope, focus here is DSG

[LekoArts]

We've hit some bigger obstacles for this (as we want to support e.g. context and GraphQL queries, too). So instead of doing a half-baked version we'll postpone this after Gatsby 4 GA and release this in a minor release in its complete state.