Feature request: opt in tsickle to closure-compiler support

This is a weird request and I could be confused but here it goes.

So there are libraries like [BabylonJS ](https://www.babylonjs.com/)that are:
- Large libraries.
- Statically typed in typescript https://github.com/BabylonJS/Babylon.js/tree/master/src
- Tree shakeable using es6 modules https://doc.babylonjs.com/features/es6_support#typescript

For a small scene, using microbuild, I got a brotli bundle of 144kb. Before es6 modules were introduced in BabylonJS and before I used microbundle, I had bundles that were over 2mbs.

In my experience, using closure-compiler could reduce this bundle by an additional 20%. However, setting up closure-compiler is a pain! I love microbundle for its zero-configuration approach and was wondering if we could get opt-in support for Typescript to [tsickle](https://github.com/angular/tsickle) to [closure-compiler](https://developers.google.com/closure/compiler/) with [ADVANCED_OPTIMIZATIONS](https://developers.google.com/closure/compiler/docs/api-tutorial3) turn on.

Getting a library like BabylonJS for just over 100k would be amazing.