Add function to fetch and update bookmarks from server

[sweep-ai[bot]]

Description

This PR adds a new function fetchBookmarks to extension/script.js that sends a GET request to the /bookmarks endpoint. The function handles the returned JSON data to add or remove bookmarks as needed.

Summary of Changes

• Added a new function fetchBookmarks to extension/script.js that sends a GET request to the /bookmarks endpoint.
• Handled the returned JSON data in the fetchBookmarks function to add or remove bookmarks based on the server data.
• Updated the fetchBookmarks function to iterate over the returned bookmarks and update the local bookmarks accordingly.
• Ensured that the function follows the same structure and conventions as the existing fetch requests in extension/script.js.
• Tested the changes to ensure the new function works as expected.

Please review and merge this PR. Thank you!

Fixes #7.

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