This commit introduces the LLEN command, which allows users to retrieve the length of a list stored in the database.
- Added `HandleLLengthCommand` function to `ListOperations.cs` to handle the LLEN command logic. This function retrieves the list associated with the given key and returns its length. If the key does not exist, it returns 0.
- Updated `Firefly.cs` to include the LLEN command in the command processing logic, mapping the "LLEN" command to the `HandleLLengthCommand` function.
