Upgrade babel to v7.2

Upgrades babel and all plugins to the latest version

Babel 7.2 release blog post here: https://babeljs.io/blog/2018/12/03/7.2.0

The primary notable change here apart from bug fixes and preset-env stat updates is the inclusion of @babel/plugin-proposal-private-methods which has now reached stage-3 proposal status.

Since we are enabling all stage-3 proposals, we now support class private methods and with the # prefix (spec here):

class Person {
  #age = 19;

  #increaseAge() {
    this.#age++;
  }

  birthday() {
    this.#increaseAge();
    alert("Happy Birthday!");
  }
}