Refactor IPFilter for readability and maintainability

This commit reorganizes the code in IPFilter.cs to enhance clarity and structure. Key changes include:
- Reorganized `using` directives and removed duplicates.
- Improved documentation formatting and consistency.
- Moved `ConcurrentHashSet<T>` class for better placement.
- Reformatted initialization of the `initial` array.
- Streamlined logic for user permissions and deny lists.
- Updated `InsertIPv4Network` and `InsertIPv6Network` methods to use null-coalescing assignment.
- Refactored `TryParseIp` for improved clarity in address parsing.
- Enhanced readability of the `ToCidrString` method.
Overall, these changes improve the code's maintainability while preserving its functionality.

Author: AslaniAk