Implement new async crawler features and stability updates

- Introduced new async crawl strategy with session management.
  - Added BrowserManager for improved browser management.
  - Enhanced documentation, focusing on storage state and usage examples.
  - Improved error handling and logging for sessions.
  - Added JavaScript snippets for customizing navigator properties.

crawl4ai/js_snippet/__init__.py

@@ -0,0 +1,15 @@
+import os, sys
+
+# Create a function get name of a js script, then load from the CURRENT folder of this script and return its content as string, make sure its error free
+def load_js_script(script_name):
+    # Get the path of the current script
+    current_script_path = os.path.dirname(os.path.realpath(__file__))
+    # Get the path of the script to load
+    script_path = os.path.join(current_script_path, script_name + '.js')
+    # Check if the script exists
+    if not os.path.exists(script_path):
+        raise ValueError(f"Script {script_name} not found in the folder {current_script_path}")
+    # Load the content of the script
+    with open(script_path, 'r') as f:
+        script_content = f.read()
+    return script_content