Add config file types to Project Structure page (#8953)

Co-authored-by: Sarah Rainsberger <sarah@rainsberger.ca>

src/content/docs/en/basics/project-structure.mdx

@@ -118,6 +118,10 @@ For help creating a new `package.json` file for your project, check out the [man
 
 This file is generated in every starter template and includes configuration options for your Astro project. Here you can specify integrations to use, build options, server options, and more.
 
+Astro supports several file formats for its JavaScript configuration file: `astro.config.js`, `astro.config.mjs`, `astro.config.cjs` and `astro.config.ts`. We recommend using `.mjs` in most cases or `.ts` if you want to write TypeScript in your config file.
+
+TypeScript config file loading is handled using [`tsm`](https://github.com/lukeed/tsm) and will respect your project's `tsconfig` options.
+
 See the [Configuring Astro Guide](/en/guides/configuring-astro/) for details on setting configurations.
 
 ### `tsconfig.json`