feat!: Add collapsible menu to Header

[alimpens]

Describe the pull request

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What

It adds a collapsible menu to Header and changes the API accordingly.

Why

Header was incomplete, we couldn't make a functional Header yet.

How

It's quite a rework of the old component. It adds a menu button to the component, and adds children for the content of the collapsible menu.

Checklist

Before submitting your pull request, please ensure you have done the following. Check each checkmark if you have done so or if it wasn't necessary:

• Add or update unit tests
• Add or update documentation
• Add or update stories
• Add or update exports in index.* files
• Start the PR title with a Conventional Commit prefix, as explained here.

Additional notes

• Header has a NarrowScreenOnlyGridCell subcomponent, maybe we can improve that name
• Header is supposed to get extra block space. ams.space.grid.md was chosen, but that looks awkward imo. I chose ams.space.grid.sm now, but that's something we should discuss.
• Should the collapsible menu have fixed top and bottom spacing?
• Spacing in general is something which should be checked by design, it wasn't always clear what it should be.

[VincentSmedinga]

Alea iacta est!