[feature: web-components]: Convert Checkbox to use ElementInternals

[chrisdholt]

Describe the issue:

This issue exists to track converting the web-component checkbox to use ElementInternals rather than the current FormAssociation behavior.

As part of this change, we should:

• Get the DOM as clean and light as possible
• Strip as much duplication of observed ARIA attributes out of the control
• Ensure the control reads as expected from assistive technology
• Add tests to validate the control participates in form association as expected by the platform.