Remove `index.d.ts`

[JamieB-gu]

Why?

Declaration files create ambient types, which appear in a global namespace across the project. We think it's preferable to avoid adding types to a global scope, and instead prefer to import them as needed.

How?

Migrate the types in the index.d.ts¹ file into normal .ts files, and import them as needed:

Footnotes

1. https://github.com/guardian/dotcom-rendering/blob/4ea272b55efa01ba88f0f5ff60f6bc9d14198147/dotcom-rendering/index.d.ts ↩

[rhiannareechaye]

@JamieB-gu I'm struggling to understand this ticket - is this a high impact ticket?