Add type definitions for API responses

[eyeseast]

We did this with add-ons. We should add coverage for the rest of the data we pull in.

It's possible this will make the src/structure folder obsolete.

[allanlasser]

Adding to this, consolidating the fixtures and mocked data handlers into the /api directory will help keep everything close-knit.

[allanlasser]

And if the API supports OpenAPI, we can generate our response types automatically:

• https://www.npmjs.com/package/openapi-typescript
• https://drf-spectacular.readthedocs.io/en/latest/readme.html

[eyeseast]

Should our types live in .d.ts files? Or in Svelte components? Somewhere else?

• https://www.typescriptlang.org/docs/handbook/declaration-files/templates/module-d-ts.html

[allanlasser]

I'll admit this isn't something I've understood well, but I think this SO answer is pretty good: https://stackoverflow.com/a/29197665/4256689

In short:

• Types that augment existing .js should go in .d.ts. This gives our imports from JS files type-checking. .d.ts is also for cases where we're not intending to compile down to working JS. Typed API responses, where there's no code we're directly referencing, seems like a good case for this kind of file.
• We can still define and export types from Svelte components when they make sense. Defining the type for an event handling function prop, for example.