[Menu buttons] nested menus design research

[laurenmrice]

Acceptance criteria

Design needs to decide if we want to allow nested menus within the Menu button component. We allow this for the context menu variant when the need for these menus seems higher for this particular use case, But is this also acceptable for Menu buttons?

• Research if the industry allows this. What is the best practice for including nested menus in a component like Menu button. Should we also allow this for Combo button?
• Consult a11y to ensure this is acceptable from an a11y perspective.
• Playback findings and make a design decision of how to move forward.

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Related: #15875

[github-actions]

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