feat(browser): add builtin browser management system

Implements a persistent browser management system that allows running a single shared browser instance
that can be reused across multiple crawler sessions. Key changes include:

- Added browser_mode config option with 'builtin', 'dedicated', and 'custom' modes
- Implemented builtin browser management in BrowserProfiler
- Added CLI commands for managing builtin browser (start, stop, status, restart, view)
- Modified browser process handling to support detached processes
- Added automatic builtin browser setup during package installation

BREAKING CHANGE: The browser_mode config option changes how browser instances are managed

docs/examples/builtin_browser_example.py

@@ -0,0 +1,86 @@
+#!/usr/bin/env python3
+"""
+Builtin Browser Example
+
+This example demonstrates how to use Crawl4AI's builtin browser feature,
+which simplifies the browser management process. With builtin mode:
+
+- No need to manually start or connect to a browser
+- No need to manage CDP URLs or browser processes
+- Automatically connects to an existing browser or launches one if needed
+- Browser persists between script runs, reducing startup time
+- No explicit cleanup or close() calls needed
+
+The example also demonstrates "auto-starting" where you don't need to explicitly
+call start() method on the crawler.
+"""
+
+import asyncio
+from crawl4ai import AsyncWebCrawler, BrowserConfig, CrawlerRunConfig, CacheMode
+import time
+
+async def crawl_with_builtin_browser():
+    """
+    Simple example of crawling with the builtin browser.
+    
+    Key features:
+    1. browser_mode="builtin" in BrowserConfig
+    2. No explicit start() call needed
+    3. No explicit close() needed
+    """
+    print("\n=== Crawl4AI Builtin Browser Example ===\n")
+    
+    # Create a browser configuration with builtin mode
+    browser_config = BrowserConfig(
+        browser_mode="builtin",  # This is the key setting!
+        headless=True            # Can run headless for background operation
+    )
+    
+    # Create crawler run configuration
+    crawler_config = CrawlerRunConfig(
+        cache_mode=CacheMode.BYPASS,  # Skip cache for this demo
+        screenshot=True,              # Take a screenshot
+        verbose=True                  # Show verbose logging
+    )
+    
+    # Create the crawler instance
+    # Note: We don't need to use "async with" context manager
+    crawler = AsyncWebCrawler(config=browser_config)
+    
+    # Start crawling several URLs - no explicit start() needed!
+    # The crawler will automatically connect to the builtin browser
+    print("\n➡️ Crawling first URL...")
+    t0 = time.time()
+    result1 = await crawler.arun(
+        url="https://crawl4ai.com",
+        config=crawler_config
+    )
+    t1 = time.time()
+    print(f"✅ First URL crawled in {t1-t0:.2f} seconds")
+    print(f"   Got {len(result1.markdown.raw_markdown)} characters of content")
+    print(f"   Title: {result1.metadata.get('title', 'No title')}")
+    
+    # Try another URL - the browser is already running, so this should be faster
+    print("\n➡️ Crawling second URL...")
+    t0 = time.time()
+    result2 = await crawler.arun(
+        url="https://example.com",
+        config=crawler_config
+    )
+    t1 = time.time()
+    print(f"✅ Second URL crawled in {t1-t0:.2f} seconds")
+    print(f"   Got {len(result2.markdown.raw_markdown)} characters of content")
+    print(f"   Title: {result2.metadata.get('title', 'No title')}")
+    
+    # The builtin browser continues running in the background
+    # No need to explicitly close it
+    print("\n🔄 The builtin browser remains running for future use")
+    print("   You can use 'crwl browser status' to check its status")
+    print("   or 'crwl browser stop' to stop it when completely done")
+
+async def main():
+    """Run the example"""
+    await crawl_with_builtin_browser()
+
+if __name__ == "__main__":
+    asyncio.run(main())