refactor(deep-crawl): reorganize deep crawling functionality into dedicated module

Restructure deep crawling code into a dedicated module with improved organization:
- Move deep crawl logic from async_deep_crawl.py to deep_crawling/
- Create separate files for BFS strategy, filters, and scorers
- Improve code organization and maintainability
- Add optimized implementations for URL filtering and scoring
- Rename DeepCrawlHandler to DeepCrawlDecorator for clarity

BREAKING CHANGE: DeepCrawlStrategy and BreadthFirstSearchStrategy imports need to be updated to new package structure

tests/20241401/test_deep_crawl.py

@@ -0,0 +1,43 @@
+import asyncio
+import time
+
+
+from crawl4ai import CrawlerRunConfig, AsyncWebCrawler,     CacheMode
+from crawl4ai.deep_crawling.bfs_strategy import BFSDeepCrawlStrategy
+
+
+async def main():
+    """Example deep crawl of documentation site."""
+    config = CrawlerRunConfig(
+        deep_crawl_strategy = BFSDeepCrawlStrategy(
+            max_depth=2,
+            include_external=False
+        ),
+        stream=False,
+        verbose=True,
+        cache_mode=CacheMode.BYPASS
+    )
+
+    async with AsyncWebCrawler() as crawler:
+        start_time = time.perf_counter()
+        print("\nStarting deep crawl in batch mode:")
+        results = await crawler.arun(
+            url="https://docs.crawl4ai.com",
+            config=config
+        )
+        print(f"Crawled {len(results)} pages")
+        print(f"Example page: {results[0].url}")
+        print(f"Duration: {time.perf_counter() - start_time:.2f} seconds\n")
+
+        print("Starting deep crawl in streaming mode:")
+        config.stream = True
+        start_time = time.perf_counter()
+        async for result in await crawler.arun(
+            url="https://docs.crawl4ai.com",
+            config=config
+        ):
+            print(f"→ {result.url} (Depth: {result.metadata.get('depth', 0)})")
+        print(f"Duration: {time.perf_counter() - start_time:.2f} seconds")
+
+if __name__ == "__main__":
+    asyncio.run(main())