Export TS interfaces so they can be used for compile-time type checking

[rain-sk]

Hello, lovely library! Thanks for all the work on this.

I'm working on a general timing library for Web Audio / Web MIDI applications, and would like to have intellisense and type checking for Interfaces that make up the webmidi package.

As an example, I can say const input = webmidi.inputs[0]. TypeScript indicates that input is of type Input, but I cannot write functions which accept the Input type as a parameter, and have to either manually copy the webmidi.d.ts to my source directory, or use the any type annotation.

I'm happy to submit a pull request if this is not a current priority!

[djipco]

I do not use TypeScript, so I'm not too familiar with it. If you submit a PR, I will happily merge it. The original webmidi.d.ts file has been graciously contributed by @mmmveggies

[rain-sk]

See: #55

[djipco]

PR #55 has been merged to address this. Thank you!

[djipco]

I tentatively added a types directory with d.ts files for all current classes (this is what the TypeScript compiler outputted). Is the usual practice to have a single index.d.ts file? In any case, if you want to give it go, be my guest!

By the way, this is in the develop branch for v3.