Add timestamp

[katydecorah]

• Adds new timestamp property where the value is an ISO string. Example: "timestamp": "2024-04-06T17:30:00.000Z",
• Updates bookmarks already in the JSON file by checking if they have timestamp if not, add one based on the date. Example:

  {
  "date": "2021-01-03",
  "site": "NYT Cooking",
  "timestamp": "2021-01-03T00:00:00.000Z", // Time is blank for previous bookmarks as we don't know, but at least gives us a consistent format
  "title": "Cornmeal Lime Shortbread Fans Recipe",
  "url": "https://cooking.nytimes.com/recipes/1021663-cornmeal-lime-shortbread-fans",
  }

• The date property remains the same.

Resolves #158