Use readonly over getter for exposing events

Fixes #4164

src/common/buffer/BufferSet.ts

@@ -20,7 +20,7 @@ export class BufferSet extends Disposable implements IBufferSet {
   private _activeBuffer!: Buffer;
 
   private _onBufferActivate = this.register(new EventEmitter<{activeBuffer: IBuffer, inactiveBuffer: IBuffer}>());
-  public get onBufferActivate(): IEvent<{activeBuffer: IBuffer, inactiveBuffer: IBuffer}> { return this._onBufferActivate.event; }
+  public readonly onBufferActivate = this._onBufferActivate.event;
 
   /**
    * Create a new BufferSet for the given terminal.

src/common/buffer/Marker.ts

@@ -16,7 +16,7 @@ export class Marker extends Disposable implements IMarker {
   public get id(): number { return this._id; }
 
   private _onDispose = new EventEmitter<void>();
-  public get onDispose(): IEvent<void> { return this._onDispose.event; }
+  public readonly onDispose = this._onDispose.event;
 
   constructor(
     public line: number