chore(gatsby): add TypeScript definitions for config & plugin APIs (#10897)
chore(gatsby): add TypeScript definitions for config & plugin APIs (#10897)

add typings support for APIs in:
* gatsby-config.js
* gatsby-node.js
* gatsby-browser.js
* gatsby-ssr.js

gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,gatsbyjs/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,ChristopherBiscardi/gatsby