Compile to ES6 #65372

This is the continuation of #47144. The goal is to down level compile the VS Code sources to modern JS so that we benefit from things like let, const, async/await and more. Things that need to be done:

• Adjust our compile scripts - reuse from Compile down to es6 #47144?
• Make sure that we don't leak modern library dependencies into the monaco-editor and layers below. This is because of our old friend IE11. Our fix for this is the double compile via tsconfig.monaco.json
• With es6 classes will be emitted as classes. This breaks extensions that are extending from our classes that are compiled to functions (see Compile down to es6 #47144 (comment))

cc João Moreno (@joaomoreno)