feat: add accessibility features to popover control

[meganaconley]

Description

Add accessibility features to the Popover control prop. This way, any element can be passed as a control, but it will still work as a button.

• tabIndex: puts normally non-tabbable elements into the normal tab flow
• aria-haspopup: indicates the button triggers a popup
• role="button": screenreader will announce element as button
• onKeyPress handler: if the control is not a button, adds enter and space as a way to trigger the popover.

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[prsdthkr]

Generally the onKeyPress works well with simple Popover controls. It gets a bit tricky with complex controls like ComboboxInput, LocalizationEditor, SearchInput because they have additional input and button within.

[jbadan]

Once the Popover is open - how do I navigate inside of it with keyboard? Or is that out of scope for this pr.
I forgot to turn keycastr on but I tabbed to the button dropdown and then was using the arrow keys - not the mouse like it looks. Bad gif sorry!

[jacobdevera]

@jbadan keyboard navigation for Popover is through tabbing in #793

I would think that a basic Popover should navigate via tabbing since it can contain any content, but it makes sense that certain compositions (Dropdown, ComboboxInput, etc.) would use arrow keys. Probably out of scope for now.

[prsdthkr]

✅ QA looks good to me on Chrome with VoiceOver.