Add support for TypeScript

[blakeembrey]

I love the simplicity of this project and want to start adopting it across the projects I maintain (replacing minimist). Providing a TypeScript definition will make adoption a lot easier by adding type safety in any strictly typed projects. Additionally, anyone using VS Code (and other editors with .d.ts support) with JavaScript will appreciate the automatic type output. It works by mapping over the "spec" type and producing the types according to the function return type (e.g. String is (x: string) => string and it'll pick out string from the return type).

[Qix-]

Thank you for the nice words, Blake :)

I am not really familiar with Typescript, but I am all for this.

Maybe @timneutkens, @leo or @javivelasco can comment on the validity (though I'd go out on a limb to say you probably know what you're talking about 😉).

[timneutkens]

What an incredible type definition, I haven't used Typescript that much, but the definition seems to be fine, so ✅

[javivelasco]

Sounds great and I need it now!