Global Using Directive

The Global Using Directive feature extends using directive syntax with an optional global keyword that can precede the using keyword. The scope of Global Using Directives spans across all compilation units in the program.

Proposal: https://github.com/dotnet/csharplang/blob/master/proposals/GlobalUsingDirective.md

Feature branch: https://github.com/dotnet/roslyn/tree/features/GlobalUsingDirective

Test plan: https://github.com/dotnet/roslyn/issues/51307