Allow CharacterCount to receive configuration via JavaScript

[romaricpascal]

The configuration spree continues! CharacterCount needs its own little behaviour when it comes to merging the data attributes and JavaScript configuration:

maxwords takes precedence over maxlength. This means we need to clear some of the JavaScript configuration if any of these options are set through data attributes to avoid a maxwords set in JavaScript always winning against a data-maxlength attribute:

new CharacterCount($element, {maxwords: 150}).init();

<div class="govuk-character-count" data-module="govuk-character-count" data-maxlength="200">
  <!-- Omitted for clarity -->
</div>

Commits should paint a step-by-step picture of how things happened.

Keeping this as draft for now as I'm on the fence of whether to rework the tests as unit tests checking that the right values end up in this.config rather than run in a browser. Opening to review with puppeteer tests so further work on internationalisation can be carried on. Tests will likely be reworked soon when/if creating a base component in 5.0.

Closes #2837

[36degrees]

🎉