fix(form-core) sync autofocus to focusable node (fixes #1775)

[Sciurus7]

What I did

1. In FocusMixin synced the autofocus attribute from the host to the focusable node.
2. Added tests to ensure the sync happens if autofocus was set on the host during render or when autofocus is added or removed from the host.

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Latest commit: 5a4b794ceea040c7bc0c09ebb88d5e7cdc6b9c4d

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[gerjanvangeest]

Hi @Sciurus7, thanks for the contribution! Somehow i missed this one completely, my excuses. Could you resolve the conflicts and add a changeset (npm run changeset)?

fix: https://github.com/ing-bank/lion/issues/1775

[Sciurus7]

Hi Gerjan, I finally managed to get my personal git account in a working state again. While trying to bring this pull request up to date I messed up a merge, so instead of trying to fix that particular mess, I branched of master again added the changes to an up to date branch and created a new pull request. This one should be up to date and has a change set. I will close this pull request accordingly.