feat(scraping): add LXML-based scraping mode for improved performance

Adds a new ScrapingMode enum to allow switching between BeautifulSoup and LXML parsing.
LXML mode offers 10-20x better performance for large HTML documents.

Key changes:
- Added ScrapingMode enum with BEAUTIFULSOUP and LXML options
- Implemented LXMLWebScrapingStrategy class
- Added LXML-based metadata extraction
- Updated documentation with scraping mode usage and performance considerations
- Added cssselect dependency

BREAKING CHANGE: None

crawl4ai/__init__.py

@@ -1,7 +1,7 @@
 # __init__.py
 
 from .async_webcrawler import AsyncWebCrawler, CacheMode
-from .async_configs import BrowserConfig, CrawlerRunConfig
+from .async_configs import BrowserConfig, CrawlerRunConfig, ScrapingMode
 from .extraction_strategy import ExtractionStrategy, LLMExtractionStrategy, CosineStrategy, JsonCssExtractionStrategy
 from .chunking_strategy import ChunkingStrategy, RegexChunking
 from .markdown_generation_strategy import DefaultMarkdownGenerator
@@ -14,6 +14,7 @@ __all__ = [
     "AsyncWebCrawler",
     "CrawlResult",
     "CacheMode",
+    "ScrapingMode",
     'BrowserConfig',
     'CrawlerRunConfig',
     'ExtractionStrategy',