Add support for JS type checking

[alexgibson]

Description

The front end team had a bit of a discussion about adding support for JS type checking to bedrock, and we agree that it would be a useful with some of our more complicated bits of JS on the site (analytics, attribution, newsletter management etc).

Options discussed so far include:

• Typescript: https://www.typescriptlang.org/
• JSDoc: https://jsdoc.app/

We haven't used JSDoc for type checking before, but it might prove simpler than Typescipt for our needs. Worth some further research.

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Success Criteria

• [ ] Ability to opt in files for JS type checking

[alexgibson]

See also: https://alexharri.com/blog/jsdoc-as-an-alternative-typescript-syntax