TypeScript definition - Use WritableStream instead of stream.Writable

[imhoffd]

Thanks for adding TypeScript types to your modules!

In Node typings, it is generally preferred to use the WritableStream interface when asking for a writable stream (not stream.Writable). This should allow people to use implementations of streams other than the built-in.

Writable itself implements the WritableStream interface: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/node/globals.d.ts#L612-L619

class Writable extends Stream implements NodeJS.WritableStream {
    ...

cc @BendingBender

[BendingBender]

@sindresorhus What do you think about it? Sounds reasonable to me.