Make streaming functionality optional

[mattt]

Follow-up to #169

Attempting to import the latest version of this library from a Next.js app produces the following error:

Module not found: Can't resolve 'stream'

The built-in Node.js stream module should be available, but they aren't. This may be an issue about Webpack 5 polyfills, but nobody has time for that — this just needs to come batteries-included.

What this PR does is two things:

• First, it adds readable-stream as an optional dependency
• Second, it updates the logic in lib/stream.js to import the Readable type from that library, falling back on the Node.js built-in. All of this is done in a try-catch, so an error won't be thrown unless the client attempts to use unsupported functionality (as opposed to simply doing import Replicate from 'replicate'.