feat: Add generic entry point for custom Redis-like clients

[matanper]

Summary

Adds a new generic entry point that allows users to provide their own Publisher/Subscriber implementations, making the library compatible with any Redis-like client (Upstash Redis, Valkey, etc.).

Changes

• New entry point: resumable-stream/generic - requires custom Publisher/Subscriber implementations
• Comprehensive tests: 4 new tests covering basic usage, resumption, and error handling
• Updated documentation: README now includes section on using custom Redis clients with example code
• Package configuration: Added exports and TypeScript definitions for the generic module

Usage Example

import { createResumableStreamContext } from "resumable-stream/generic";
import type { Publisher, Subscriber } from "resumable-stream/generic";

// Create your custom adapters
const publisher: Publisher = {
  connect: async () => { /* ... */ },
  publish: async (channel, message) => { /* ... */ },
  set: async (key, value, options) => { /* ... */ },
  get: async (key) => { /* ... */ },
  incr: async (key) => { /* ... */ },
};

const subscriber: Subscriber = {
  connect: async () => { /* ... */ },
  subscribe: async (channel, callback) => { /* ... */ },
  unsubscribe: async (channel) => { /* ... */ },
};

const streamContext = createResumableStreamContext({
  waitUntil: null,
  publisher,
  subscriber,
});