- Add Slack Socket Mode handler in server.py
- Create start_socket_mode function in slack/app.py
- Add example script for testing Slack websocket handler

Introduce Slack Socket Mode support alongside existing HTTP handler.
This allows for real-time message processing using websockets when
SLACK_WEBSOCKET_TOKEN is set. The HTTP handler remains active when
only SLACK_BOT_TOKEN and SLACK_SIGNING_SECRET are provided.

- Update comment to mention "SLACK_WEBSOCKET_TOKEN" instead of
  "SLACK_APP_TOKEN"

Fix a typo in the comment describing the environment variable check
for the Slack socket handler. This change aligns the comment with
the actual code implementation, improving code readability and
preventing potential confusion.

- Update docstring to mention "SLACK_WEBSOCKET_TOKEN" instead of
  "SLACK_APP_TOKEN"

Correct the documentation in the `start_socket_mode` function to
accurately reflect the environment variable used for Slack
authentication. This change aligns the docstring with the actual
code implementation, improving clarity and preventing potential
confusion for developers.