Readonly helps with type guards and code comprehension

Const is preferred for type guards

We compile using the alwaysStrict flag so these directives are not needed

This part removes most `use strict` directives that are right after the file header

* have explicit methods for changing the scroll position (delayed/smooth vs now/direct) that bubble up to the editor internals.
* have all other clients of ScrollableElement not use smooth scrolling

Do not use translate3d in scrollbars, use will-change for scrollbar sliders instead (Microsoft/monaco-editor#426)