Refactor JSONCookie function for clarity and compliance

[Ayoub-Mabrouk]

• Update JSDoc to clarify behavior and return types.
• Replace str.substr(0, 2) with str.startsWith to check for the prefix 'j:'.
• Simplify error handling by removing unused catch parameter.
• Maintain functionality for parsing JSON cookies, returning undefined for invalid inputs.

[UlisesGascon]

same as https://github.com/expressjs/cookie-parser/pull/119#issuecomment-2451729449