Fix completion text for clients which don’t support TextEdit

We could set the `Label` to `CompletionText` as the fallback, but that
then makes the list of completions in fancier clients not as
accurate (since PowerShell explicitly provides `ListItemText` for this
purpose). Instead, we set usually unused field `InsertText` with the
same information we provided for more advanced clients in `TextEdit`.

Author: andyleejordan

src/PowerShellEditorServices/Services/TextDocument/Handlers/CompletionHandler.cs

@@ -185,7 +185,10 @@ internal static CompletionItem CreateCompletionItem(
                 // Retain PowerShell's sort order with the given index.
                 SortText = $"{sortIndex:D4}{result.ListItemText}",
                 FilterText = result.CompletionText,
-                TextEdit = textEdit // Used instead of InsertText.
+                // Used instead of Label when TextEdit is unsupported
+                InsertText = result.CompletionText,
+                // Used instead of InsertText when possible
+                TextEdit = textEdit
             };
 
             return result.ResultType switch