Use | undefined instead of ? for some readonly properties on objects that we control

For #124362

This includes:

- Event objects
- Context objects passed to providers
- Managed objects such as `TextEditor`

src/vs/workbench/api/common/extHost.protocol.ts

@@ -1477,9 +1477,9 @@ export interface ISignatureHelpDto {
 
 export interface ISignatureHelpContextDto {
 	readonly triggerKind: modes.SignatureHelpTriggerKind;
-	readonly triggerCharacter?: string;
+	readonly triggerCharacter: string | undefined;
 	readonly isRetrigger: boolean;
-	readonly activeSignatureHelp?: ISignatureHelpDto;
+	readonly activeSignatureHelp: ISignatureHelpDto | undefined;
 }
 
 export interface IInlayHintDto {

src/vs/workbench/api/common/extHostTypes.ts

@@ -1442,7 +1442,7 @@ export enum CompletionTriggerKind {
 
 export interface CompletionContext {
 	readonly triggerKind: CompletionTriggerKind;
-	readonly triggerCharacter?: string;
+	readonly triggerCharacter: string | undefined;
 }
 
 export enum CompletionItemKind {