Fix conditional import / export of streaming functionality

[mattt]

Resolves #179

The stream method introduced by #169 requires a readable stream interface. #172 introduced a conditional import, whereby the library attempts to load the built-in stream module and attempts to fall back on the readable-stream polyfill. However, something about that seems to confuse Next.js (during tree-shaking?), causing it to report an import error in stream when tracing unrelated errors.

This PR refactors this in the following ways:

• Use import instead of require
• Export createStream instead of Stream class; the function can throw if Readable isn't found
• Import node:stream instead of stream to help distinguish use of core module (this was introduced in Node.js 14)

[paramaggarwal]

Looks good, let's go ahead with merge. Sharing a copy of the error mentioned in #179 as it is an image there and not searchable.

On Next.js 14.1:

./node_modules/replicate/lib/stream.js
Module not found: Can't resolve 'stream' in '/Users/param/Code/poc/node_modules/replicate/lib'
Did you mean './stream'?
Requests that should resolve in the current directory need to start with './'.
Requests that start with a name are treated as module requests and resolve within module directories (node_modules).
If changing the source code is not an option there is also a resolve options called 'preferRelative' which tries to resolve these kind of requests in the current directory too.

Current workaround: install readable-stream npm package.

[mattt]

Closing in favor of #214