Implement onRender in xterm-headless

This effectively enabled implementing an efficient renderer on top
of xterm-headless. See #5392

src/headless/public/Terminal.test.ts

@@ -202,6 +202,15 @@ describe('Headless API Tests', function (): void {
       strictEqual(callCount, 2);
     });
 
+    it('onRender', async () => {
+      const calls: { start: number, end: number }[] = [];
+      term.onRender(e => calls.push(e));
+      await writeSync('foo');
+      deepStrictEqual(calls, [{ start: 0, end: 0 }]);
+      await writeSync('\n\nbar');
+      deepStrictEqual(calls, [{ start: 0, end: 0 }, { start: 0, end: 2 }]);
+    });
+
     it('onScroll', async () => {
       term = new Terminal({ rows: 5 });
       const calls: number[] = [];

src/headless/public/Terminal.ts

@@ -77,6 +77,7 @@ export class Terminal extends Disposable implements ITerminalApi {
   public get onCursorMove(): Event<void> { return this._core.onCursorMove; }
   public get onData(): Event<string> { return this._core.onData; }
   public get onLineFeed(): Event<void> { return this._core.onLineFeed; }
+  public get onRender(): Event<{ start: number, end: number }> { return this._core.onRender; }
   public get onResize(): Event<{ cols: number, rows: number }> { return this._core.onResize; }
   public get onScroll(): Event<number> { return this._core.onScroll; }
   public get onTitleChange(): Event<string> { return this._core.onTitleChange; }