Support inmemory schema

Used by the `MonacoLanguageClient`. Because it has no real file backing, we had
to check that no file was returned in the prior commit.

Author: andyleejordan

src/PowerShellEditorServices/Services/Workspace/WorkspaceService.cs

@@ -192,6 +192,7 @@ public bool TryGetFile(DocumentUri documentUri, out ScriptFile scriptFile)
             {
                 // List supported schemes here
                 case "file":
+                case "inmemory":
                 case "untitled":
                 case "vscode-notebook-cell":
                     break;