feat: export unwrapProxy and IS_PROXIED from public API

[apankov1]

Summary

Exports unwrapProxy and IS_PROXIED from the main package entry point.

Problem

The unwrapProxy function was added in PR #5 (cherry-pick of cloudflare#100) but only used internally for RPC auto-unwrapping.

Consumers need direct access to unwrapProxy for:

• structuredClone() calls on @persist objects
• Manual serialization in non-RPC contexts
• Caching operations

Solution

Export both symbols from index.ts:

• unwrapProxy<T>(value: T): T - Recursively strips proxy wrappers
• IS_PROXIED - Symbol to detect proxied objects

Usage

import { unwrapProxy } from '@cloudflare/actors';

// Before: DataCloneError
const cached = structuredClone(this.persistedField);

// After: Works correctly
const cached = structuredClone(unwrapProxy(this.persistedField));