WIP: Convert all graphic promotion files to TypeScript

[dominicfraser]

This PR depends on tooling updates in https://github.com/Skyscanner/backpack/pull/2527, but demonstrates how the Graphic Promotion component could be updated to TypeScript.

It changes test and Storybook files alongside the implementation, and fixes errors in those files that TS showed.

Remember to include the following changes:

• [ ] UNRELEASED.md (See How to write a good changelog entry)
• [ ] README.md
• [ ] Tests
• [ ] Docs (either update backpack-docs now, or create a follow up ticket)