Fixes #2: Improve map functionality to load more stops on pan and zoom

[tarunsinghofficial]

Fixed #2

Tasks done:

• Added a debounce function to ensure the loadStopsAndAddMarkers function is not called more frequently than once every 300 milliseconds.
• The loadStopsAndAddMarkers function fetches stops for the current center of the map and adds markers after checking for duplicates.
• The markerExists function checks if a marker for a specific stop ID already exists.

Screenshot: