Necessary updates for the new ternary operator added in PowerShell language

[daxian-dbw]

The ternary operator will soon be added to the PowerShell language. Necessary updates may be needed for the vscode-powershell extension:

1. New AST type TernaryExpressionAst is added. Changes are made to the visitors in PowerShell. The visitors in powershell extension may need to be updated too.
2. The new ternary operator may cause syntax colorization issue.

/cc @rjmholt @TylerLeonhardt

[TylerLeonhardt]

The new ternary operator may cause syntax colorization issue.

Can you open an issue here:
https://github.com/PowerShell/EditorSyntax

[daxian-dbw]

Done: PowerShell/EditorSyntax#184