DEV: updates js transpiler to use babel 7

[jjaffeux]

Updates our js transpiler code to use Babel 7.11.6

List of changes in this commit:

• Updates plugins, babel plugins all have a new version which doesn't contain -es2015- anymore
• Drops transform-es2015-classes this plugin shouldn't be needed now that we don't support IE
• Drops check-es2015-constants, checking constants is now part of babel and the check-constants plugin is deprecated. As a result the behavior slightly changed, and is now wrapping every const call in a readOnlyError function which would throw if assigned a new value. This explains the modified spec.
• Adds proposal-optional-chaining

const obj = {
  foo: {
    bar: {
      baz: 42,
    },
  },
};

const baz = obj?.foo?.bar?.baz; // 42

• Adds proposal-json-strings

// IN
const ex = "before
after";
//                ^ There's a U+2028 char between 'before' and 'after'


// OUT
const ex = "before\u2028after";
//                ^ There's a U+2028 char between 'before' and 'after'

• Adds proposal-nullish-coalescing-operator

var object = {};
var foo = object.foo ?? "default"; // default

• Adds proposal-logical-assignment-operators

let a;
let b = 2;
a ||= b; // 2

• Adds proposal-numeric-separator

let budget = 1_000_000_000_000;
console.log(budget === 10 ** 12); // true

• Adds proposal-object-rest-spread https://babeljs.io/docs/en/babel-plugin-proposal-object-rest-spread

let { x, y, ...z } = { x: 1, y: 2, a: 3, b: 4 };
console.log(x); // 1
console.log(y); // 2
console.log(z); // { a: 3, b: 4 }

• Adds proposal-optional-catch-binding https://babeljs.io/docs/en/babel-plugin-proposal-optional-catch-binding

try {

} catch {

} finally {
  // ensures finally is available in every browsers
}

• Adds improved regex support for firefox through (transform-dotall-regex](https://babeljs.io/docs/en/next/babel-plugin-transform-dotall-regex.html) and (proposal-unicode-property-regex](https://babeljs.io/docs/en/babel-plugin-proposal-unicode-property-regex)

• Adds for-await...of support for Edge through (proposal-async-generator-functions](https://babeljs.io/docs/en/babel-plugin-proposal-async-generator-functions)

[jjaffeux]

The backend error seems legit and related: https://github.com/discourse/discourse/pull/10627/checks?check_run_id=1085914427

Will investigate.