Feature(typescript): Migrate codebase to typescript

[EandrewJones]

Problem

There are an unknown number of lurking type issues (see, for example, #441) all of which would be solved by migrating to typescript. What's more, we employ a strategy pattern in a number of places such as the authHeaderCallbacks and to enforce the strategy we write explicit typechecks in javascript.

Both of these point to the need/desire for type support to improve the developer experience.

Proposal

Migrate the code base to typescript in a single push.

Risks

• This will be a large PR.
• You never know whether one of your dependencies won't play nice with TS.
• Typescript also means that new contributors and developers will need to adhere to the type system, assuming we enforce this via pre-commit hooks/linters.