Add accordion block

[jffng]

About
An accordion is a common UX pattern with specific semantics. I based this block's structure and markup on a few examples:

https://www.w3.org/WAI/ARIA/apg/patterns/accordion/
https://designsystem.digital.gov/components/accordion/
https://www.aditus.io/patterns/accordion/

I used the block as a way to get familiar with the interactivity API, experimented with a few different approaches, but I'd love to get some feedback before moving forward.

Screenshots

Editor	View

To Test

• Check out the PR into your plugins directory, build it (npm i && npm run build), and activate the plugin.
• Verify the block is working generally working as expected.

To-Do

• Enable a setting for only one accordion item to be open at a time
• Enable "Open by default" setting
• Editor: show content only when item is selected or when "Open by default" is toggled on
• Add a setting for the icon size
• Add animation transition settings
• Increase button hit area while still allowing for padding controls to work
• Add default icon behavior
• Add a few icon options
• Enable alignment
• Add default animation transition that respects users motion preferences
• Add polyfill for latest react runtime
• Allow heading spacing to be adjusted via block space
• Add setting for icon position

[jffng]

Closing in favor of implementing in #16 or Core.