[INFRA] Convert gatsby-config.js to gatsby-config.ts

[csouchet]

Support added in gatsby@4.9.0

You can import the type GatsbyConfig to type your config object. Please note: There are currently no type hints for plugins and you’ll need to check the current limitations and see if they apply to your gatsby-config.ts file.

For more information, see https://www.gatsbyjs.com/docs/how-to/custom-configuration/typescript/#gatsby-configts.