Add Typescript definitions

[BehindTheMath]

No description provided.

[BehindTheMath]

Never mind.

According to Typescript's documentation, if the package is not written in Typescript, it's preferable to add the type definitions to the DefinitelyTyped repo.

[BehindTheMath]

Actually, DefinitelyTyped's recommendation is:

If you are the library author, or can make a pull request to the library, bundle types instead of publishing to DefinitelyTyped.

So I'm reopening this PR to add the definitions here.

[robinnorth]

LGTM, though I'm not a Typescript user. You might want to add a note to the readme to the effect that any PRs should ideally include updated Typescript definitions if necessary, in addition to passing tests.

[BehindTheMath]

I'm going to try to figure out if there's an easy way to write tests for this. Then we could include that in the testing workflow.

[robinnorth]

Great, sounds good to me. Feel free to merge when you're happy!

[BehindTheMath]

I haven't added any tooling for actually running or automating the tests. To run it, Typescript would need to be installed, and I don't want to deal with that now.

To run it manually, install Typescript (either locally or globally), and then run:

tsc --noEmit ./tests/test.ts