chore(site): isolated playground base code

[ala-n]

Example of usage:

<uip-root>
  <script type="text/html" uip-snippet label="Mixin example">
    The time is: <time time-inject-mixin></time>
  </script>
  <script type="text/html" uip-js-snippet label="Mixin example">
    import {ESLMixinElement} from '@exadel/esl';

    class TimeInjectMixin extends ESLMixinElement {
      static get is() {
        return 'time-inject-mixin';
      }

      connectedCallback() {
        super.connectedCallback();
        this.update();
      }

      update() {
        this.$host.innerHTML = new Date().toLocaleTimeString();
        this._timeout = setTimeout(() => this.update(), 1000);
      }
    }
    TimeInjectMixin.register();
  </script>

  <uip-snippets class="uip-toolbar" dropdown-view="@xs"></uip-snippets>
  <uip-preview isolation style="height: 50px"></uip-preview>
  <uip-editor label="Source code (HTML)" collapsible copy></uip-editor>
  <uip-editor script label="Source code (JS)" collapsible copy></uip-editor>
</uip-root>

[codeclimate]

Code Climate has analyzed commit 12f6ae9 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (50% is the threshold).

This pull request will bring the total coverage in the repository to 64.6% (0.0% change).

View more on Code Climate.

[ala-n]

🎉 This PR is included in version 5.0.0-beta.10 🎉

The release is available on:

• npm package (@beta dist-tag)
• GitHub release

Your semantic-release bot 📦🚀

[ala-n]

🎉 This PR is included in version 4.15.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀