Follow-up work for the Block Directory

[noisysocks]

We opted to merge #17431 behind an experimental flag with some outstanding issues, listed here:

• API: Make it more "RESTful"
• API: Ensure the directory respects canInsertBlockType, (e.g. when the selected block has a template lock like the column block does, or only specific child blocks are allowed).
• UI: Consider removing 'with an average rating of 0' copy
• A11y: Determine better alt text for block icons
• A11y: Improve 'Add' button copy
• UI: Consider alt text for icons in search results
• UI: Fix onMouseEnter/onMouseLeave
• UI: Use inline notices
• Refactor: Replace Slot/Fill with a more useful abstraction
• Refactor: Split reducer into seperate reducers
• API: Use canUse to check permissions
• Insertion: Make loadScript async
• Insertion: Double check handling of object form
• Insertion: Clarify 0 in resolve( 0 )
• Refactor: Move uninstallation code to own component
• API: Use new WP_Error
• Insertion: Fix handling of script dependencies
• Flags: Add tree shaking
• Tests: Add E2E tests for the Block Directory flow
• Tests: Add unit tests for the selectors, actions, reducers, etc.
• Tests: Add unit tests for the new API controller

cc. @tellyworth @dd32 @talldan @ck-lee

[StevenDufresne]

Just noting that I'm checking the boxes for items represented elsewhere in a PR or another issue with the aim of closing this out. 🧹

[StevenDufresne]

I'm closing this issue since everything that is directly related to the original PR is completed save for the Slot/Fill abstraction which is not specific to the block directory and the source code has been refactored a few times already.