Enhance XML docs, null safety, and add CreateLink method

- Added XML documentation comments to `IShellLinkW` methods and `CreateLink` method.
- Changed `LinkDisplayMode` visibility from `public` to `internal`.
- Updated `LargeIcon` and `SmallIcon` properties to return nullable `Icon` types.
- Modified `GetIcon` method to return a nullable `Icon` type and use nullable `IntPtr` arrays.
- Added null checks and exception handling in various properties to handle null `linkA` and `linkW`.
- Renamed `UnManagedMethods` to `UnmanagedMethods` and updated `ExtractIconEx` method to use nullable `IntPtr` arrays.
- Added `CreateLink` method in `WinApi.cs` for creating shortcut files with detailed exception handling and validation.