Cannot use global using directive.

[sandichhuu]

Describe the bug
"global using directive" not work.

To Reproduce
Steps to reproduce the behavior:

1. Make a script contain this line:
   global using MB = UnityEngine.MonoBehaviour
2. Go back to Unity Editor and got error: "A namespace cannot directly contain members such as fields or methods"

Expected behavior
https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-directive

Environment (please complete the following information):

• Version: 1.5.0
• Platform: Unity Editor
• Unity version: 2021.3.5
• Build options: Mono + .Net 4.x