Make TypeScript peer dependency optional

[raypatterson]

Thanks for this library!

I'm noticing that because TypeScript is a peer dependency that when this library is used as dependency in a package that bundles to JavaScript, when that package is installed, TypeScript is also installed, even when the project does not use TypeScript.

I'm guessing the idea is to try to ensure compatibility with the types, but it's ~40MB to install TypeScript that might be avoided by making this peer dependency optional.