Avoid teleporting nodes in SlotTargetMixin

[tomivirkki]

SlotTargetMixin currently teleports nodes assigned to a specific slot. For example, if you create a <vaadin-checkbox> with plain-text -content, the content is teleported inside a generated <label slot="label">. This works to an extent with static text, but with dynamically changing text eg. in a Lit-template, teleporting user-defined content like this doesn't work:

<div id="root"></div>
<button id="render">Render</button>

<script type="module">
  import '@vaadin/checkbox';
  import {render, html} from 'lit';

  const wrapper = document.querySelector('#root');
  const button = document.querySelector('#render');

  let i = 0;
  button.addEventListener('click', () => {
    render(html`
      <vaadin-checkbox>Label ${++i}</vaadin-checkbox>
  `, wrapper);
  });
</script>

[vursen]

Useful links related to the issue

The following PR is where SlotTargetMixin is introduced:

• feat: add SlotTargetMixin for checkbox and radio-button #2482

The following PRs are where SlotTargetMixin comes to use:

• refactor!: update checkbox to use slotted input #2539
• refactor!: update radio-button to use slotted input #2572