Add isRetrigger field to signature help context

#54972

Instead of having a generic `Retrigger` reason, add a new field on the `SignatureHelpContext` that tracks if this was a retrigger or not. This allows retrigger for all the different trigger reasons, including invoke.

Replace the old `Retriggger` trigger reason with `ContentChange` which tracks cursor movement and text updates.

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

@@ -1115,7 +1115,7 @@ export class SignatureHelp {
 export enum SignatureHelpTriggerReason {
 	Invoke = 1,
 	TriggerCharacter = 2,
-	Retrigger = 3,
+	ContentChange = 3,
 }
 
 export enum CompletionTriggerKind {