[Proposal] Add ability to declare global usings for namespaces, types and aliases by using a command line switch

Motivation - provide shared context for the program to reduce repetition of common using directives across all source files. For example, ```using System;```, etc. VB compiler supports that for many years. 

Specification: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/GlobalUsingDirective.md
