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
• [x] Add default icon behavior
• [x] Add a few icon options
• [x] Enable alignment
• [x] Add default animation transition that respects users motion preferences
• [x] Add polyfill for latest react runtime
• [x] Allow heading spacing to be adjusted via block space
• [x] Add setting for icon position

[jffng]

Closing in favor of implementing in #16 or Core.