Add gatsby-plugin-typegen instead of codegen

[item4]

I think website of TypeScript must be type-safe, right?
But, there is few type definition only.
Almost props of components use (props: any). 😱
So... I make type definition file automatically with gatsby-plugin-typegen.
This plugin support autogenerated type-def file with gatsby together.

Please read this PR and consider merge.

[msftclas]

All CLA requirements met.

[orta]

Ha nice, I only removed it a day or two ago because I introduced the shared fragments and the old one couldn't handle it!

If you can add the generated types to gitignore then this is good to go thanks! Wooo

[orta]

GitHub is having issues, I'm gonna manually merge this! Thanks!

[cometkim]

Hey @item4 and @orta, I'm author of the plugin. I'm really happy to see that the official TypeScript webpage uses my plugins, appreciates all of your work.

The plugin is currently actively developing. so If you have any trouble to use the plugin, please give feedback.

[orta]

I just changed this to:

import { AllSitePageFragment } from "../__generated__/gatsby-types";
export type AllSitePage = AllSitePageFragment["allSitePage"];

in case you don't know about the type accessor feature in TS

[item4]

oh. It's nice. thanks your kindness :)

[orta]

👍 it's working so far

[Urigo]

that's awesome!
We also use that plugin (we are a bit biased as it's using https://github.com/dotansimha/graphql-code-generator under the hood ;) )

@orta if you think this is good, I think it would be a good idea to update your amazing blog post on Gatsby and Typescript.
I assume a lot of people will read it and use it as a resource!