Move a11y files to @yoast/a11y

[LisanneKluitmans]

Summary

We want to have structured packages for all our components. To achieve this, we need to move the a11y files to a new package.

Explanation

These a11y files need to be moved to the new @Yoast/a11y package: javascript/packages/yoast-components/a11y/

Technical decisions

The version of the new package needs to be 0.1.0

Feedback?

[diedexx]

There's an a11y notice component that could also be moved. We're not sure where this is located. either yoast-components/shared or @yoast/components