Add TypeScript Compatibility/Support

[patricia-tonella]

Hi there! 👋🏻

I'm currently migrating my Gatsby config files from .js to .jsx and .ts. It appears that the latest version of gatsby-plugin-robots-txt plugin does not currently offer TypeScript support. Given that Gatsby now fully supports TypeScript for config files, I was wondering if there are any plans to upgrade the plugin to support TS as well?

I'd be happy to open a pull request with the necessary changes, or collaborate if someone else is already working on it.

Thank you!