feat: Remove getCanvasManager, export CanvasManager class directly

This simplifies the code a bit by exporting the CanvasManager directly. With [ReplayCanvas](getsentry/sentry-javascript#10112), we can rely on it for complex setup, but keeps it simple for our users.

packages/rrweb/src/index.ts

@@ -26,7 +26,12 @@ export { record, Replayer, utils, canvasMutation };
 
 export { deserializeArg } from './replay/canvas/deserialize-args';
 
-export { takeFullSnapshot, mirror, freezePage, addCustomEvent } from './record';
+export {
+  takeFullSnapshot,
+  mirror,
+  freezePage,
+  addCustomEvent,
+} from './record';
 
 export { CanvasManager } from './record/observers/canvas/canvas-manager';
 export type { CanvasManagerConstructorOptions } from './record/observers/canvas/canvas-manager';