feat: add SlotTargetMixin for checkbox and radio-button

[vursen]

Description

This PR introduces SlotTargetMixin that forwards every node from a source slot to a target element.

The SlotTargetMixin is based on the prototype.

The SlotTargetMixin is needed by the future SlotLabelMixin to move the content from the default slot (a conventional way to define a label for a checkbox) to the slotted <label> element.

Part of #2211

Type of change

• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs-beta/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

[sonarcloud]

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[vaadin-bot]

This ticket/PR has been released with platform 22.0.0.alpha4 and is also targeting the upcoming stable 22.0.0 version.