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

test/PowerShellEditorServices.Test.Shared/Completion/CompleteCommandFromModule.cs

@@ -26,6 +26,7 @@ internal static class CompleteCommandFromModule
             Kind = CompletionItemKind.Function,
             Detail = "", // OS-dependent, checked separately.
             FilterText = "Get-Random",
+            InsertText = "Get-Random",
             Label = "Get-Random",
             SortText = "0001Get-Random",
             TextEdit = new TextEdit

test/PowerShellEditorServices.Test.Shared/Completion/CompleteCommandInFile.cs

@@ -23,6 +23,7 @@ internal static class CompleteCommandInFile
             Kind = CompletionItemKind.Function,
             Detail = "",
             FilterText = "Get-Something",
+            InsertText = "Get-Something",
             Label = "Get-Something",
             SortText = "0001Get-Something",
             TextEdit = new TextEdit

test/PowerShellEditorServices.Test.Shared/Completion/CompleteNamespace.cs

@@ -23,6 +23,7 @@ internal static class CompleteNamespace
             Kind = CompletionItemKind.Module,
             Detail = "Namespace System.Collections",
             FilterText = "System.Collections",
+            InsertText = "System.Collections",
             Label = "Collections",
             SortText = "0001Collections",
             TextEdit = new TextEdit

test/PowerShellEditorServices.Test.Shared/Completion/CompleteTypeName.cs

@@ -23,6 +23,7 @@ internal static class CompleteTypeName
             Kind = CompletionItemKind.TypeParameter,
             Detail = "System.Collections.ArrayList",
             FilterText = "System.Collections.ArrayList",
+            InsertText = "System.Collections.ArrayList",
             Label = "ArrayList",
             SortText = "0001ArrayList",
             TextEdit = new TextEdit

test/PowerShellEditorServices.Test.Shared/Completion/CompleteVariableInFile.cs

@@ -23,6 +23,7 @@ internal static class CompleteVariableInFile
             Kind = CompletionItemKind.Variable,
             Detail = "", // Same as label, so not shown.
             FilterText = "$testVar1",
+            InsertText = "$testVar1",
             Label = "testVar1",
             SortText = "0001testVar1",
             TextEdit = new TextEdit