Add Babel to transform ES2020 public and static class fields

[colinrotherham]

This PR adds Babel transpilation as decided in:

• Add decision record about JavaScript browser compatibility govuk-design-system-architecture#25

I've added transforms for two ES2020 features back to ES2015—without polyfills or helper functions

Public static fields

After merging Migrate JavaScript to ES2015 classes we were left with "optional" class properties appended in the constructor that would be clearly defined as static in ES2020

Before

export class I18n {
  constructor() {
    // ...
  }
}

I18n.pluralRulesMap = {}
I18n.pluralRules = {}

After

export class I18n {
  constructor() {
    // ...
  }
  
  static pluralRulesMap = {}
  static pluralRules = {}
}

Public fields

We also have "optional" class properties appended in the constructor that would be public class fields

Before

export class Radios {
  constructor($module) {
    if (!($module instanceof HTMLElement)) {
      return this
    }
  
    this.$module = $module
    this.$inputs = $module.querySelectorAll('input[type="radio"]')
  }
}

After

Both $module and $inputs can be declared as class fields (with default values if required)

export class Radios {
  $module
  $inputs

  constructor($module) {
    if (!($module instanceof HTMLElement)) {
      return this
    }
  
    this.$module = $module
    this.$inputs = $module.querySelectorAll('input[type="radio"]')
  }
}

[domoscargin]

Diff looks good to me - time to use all the shiny things!

As noted above, we're holding off merging this till Exit This Page gets merged.

[colinrotherham]

Thanks @domoscargin

My only concern (now) is it's slightly jarring seeing JSDoc comments moved to the bottom of the class

@36degrees @romaricpascal Any further thoughts on removing comments to keep KB size down? Hadn't realised how easy it was with Babel generatorOpts: { comments: false } and they're all gone

Comments can use @preserve to keep important blocks too:

• Configure Babel to remove non-public comments #3958

Seeing ~40-50% drop in file size KB

[romaricpascal]

Neat ! And thanks for the review @domoscargin 🙌🏻 I'm pretty keen to remove unnecessary comments (especially given the gains highlighted by the spike), but let's discuss that as a separate PR so we can have this merged on its own 😊

[romaricpascal]

question Noticed while looking at #3958 that this list is missing the mql field, which is still first used in setupResponsiveChecks a little further. Is it intentional, or just that is was missed among all the other fields? 😊

[colinrotherham]

@romaricpascal Nice spot, yeah it was missed. Added it 👍

[colinrotherham]

@romaricpascal @domoscargin Updated with Exit this page from #3953

[romaricpascal]

Transformation looks fine to me on the file for Exit this page as well 🥳