feat: add KeyboardMixin and make other mixins use it

[vursen]

Description

The ActiveMixin, ClearButtonMixin and future CheckedMixin all need to handle the keyup and keydown events.

Along with that, the prototype of the new Checkbox component supposes it to inherit both ActiveMixin and CheckedMixin. This being so, these mixins will end up in the same scope and there may happen a naming collision (in case both define a _onKeyDown method, for example).

So it was decided to introduce KeyboardMixin that is responsible for subscribing to the keyboard events while leaving actual implementation of the event handlers to the client (a component or another mixin).

See the prototype: https://github.com/vaadin/component-mixins/blob/master/packages/keyboard-mixin/keyboard-mixin.ts

Part of #2210.

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.

[web-padawan]

LGTM 👍

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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[vaadin-bot]

This ticket/PR has been released with platform 22.0.0.alpha3 and is also targeting the upcoming stable 22.0.0 version.