Update types relevant to key input events and their documentations

- Added the XML documentation for `Presentation.VirtualKeyCode`.
- Updated the XML documentation for `Presentation.KeyModifierFlags`.
- Removed `LMenu` and `RMenu` from `Presentation.VirtualKeyCode` since
  `winit` will generate `[LR]Alt` instead of them in further versions:
  <rust-windowing/winit#551>.

Ngs.Engine.Core/Engine/Presentation/KeyModifierFlags.cs

@@ -12,22 +12,22 @@ namespace Ngs.Engine.Presentation {
     [Flags]
     public enum KeyModifierFlags {
         /// <summary>
-        /// The "Shift" key.
+        /// The shift key.
         /// </summary>
         Shift = 1 << 0,
 
         /// <summary>
-        /// The "Control" key.
+        /// The control key.
         /// </summary>
         Control = 1 << 1,
 
         /// <summary>
-        /// The "Alt" key.
+        /// The alt key.
         /// </summary>
         Alt = 1 << 2,
 
         /// <summary>
-        /// The "Meta" key (e.g., the Windows logo key in Windows, Command in macOS).
+        /// The meta key (e.g., the Windows logo key in Windows, Command in macOS).
         /// </summary>
         Meta = 1 << 3,
     }