Polyfill missing std lib fns for module browsers

[Timer]

This pull request creates a new @next/polyfill-module that fills in the missing gaps between various nomodule browsers.

It supports:

• String#trimStart / String#trimEnd
• Symbol#description
• Array#flat / Array#flatMap
• Promise#finally (previously polyfilled, now relocated)

---

Fixes #11173

[minhtule]

@ijjk This polyfill doesn't work properly on Edge 44 (#18213) and Safari 11 (#17825). We can't upgrade to 9.5.4 to fix the security alert and also 10.0 today... Does Next.js need this polyfill internally? If not, could we remove it for now?

[Timer]

Can you send a PR to fix it?

[minhtule]

Yes, I can open a PR to fix the issue on Edge 44 with a small change

Object.defineProperty(Symbol.prototype, 'description', {
  get: function get() {
    const result = /\((.+)\)/.exec(this)[1];
    return result ? result[1] : undefined;
  },
})

However, this still doesn't handle the other issue with Safari 11 (#17825). Core-js es.symbol.description polyfill seems to be more complete (handling all browser) but I'm not sure if we want to bring all that code into Next.js. I would suggest that we remove this problematic polyfill from Next.js. Whoever needs it can polyfill it manually.

@Timer WDYT?

[developit]

Core-js does not polyfill symbol.description, it polyfills Symbols themselves (in their entirety). This works:

if (!('description' in Symbol.prototype)) {
  Object.defineProperty(Symbol.prototype, 'description', {
    get: function get() {
      return /\((.+)\)/.exec(this.toString())[1]
    },
  })
}