Semantic Highlighting for Powershell in VS Code

[justinytchen]

Summary of the new feature
Leverage the tokens used by the Powershell AST for more accurate highlighting

Proposed technical implementation details (optional)
Create a new handler in PowerShell Editor Services to handle requests to identify semantic tokens
Find/Create a mapping between PowerShell tokens and Semantic tokens VS Code expects

PowerShell/PowerShellEditorServices#1328
#2834