refactor: migrate gatsby-config to TypeScript

[nickspaargaren]

Gatsby now supports TypeScript for the config file. This commit updates the config to make use of the GatsbyConfig type.
https://www.gatsbyjs.com/docs/how-to/custom-configuration/typescript/#gatsby-configts

[jpedroschmitz]

Thanks a lot, @nickspaargaren! Could you also migrate the gatsby-node.js file? I think it would look something like this:

import TsconfigPathsPlugin from 'tsconfig-paths-webpack-plugin';

import type { GatsbyNode } from 'gatsby';

export const onCreateWebpackConfig: GatsbyNode['onCreateWebpackConfig'] = ({
  actions,
}) => {
  actions.setWebpackConfig({
    resolve: {
      plugins: [new TsconfigPathsPlugin()],
    },
  });
};

[nickspaargaren]

Sure, I'll create a new PR later today because of the branch naming, good catch!

[jpedroschmitz]

Don't worry about the branch name! It's ok to add this change to this PR!