Block Library: enable slash inserter for heading, list, and quote

[gwwar]

Follow up to #35196, this enables the slash inserter in the heading, list and quote blocks. Changes here provide folks more ways to transform a block. Do the blocks here feel like good matches for the slash inserter? Are there any other blocks we'd like to enable the slash inserter for?

CleanShot.2021-10-05.at.09.19.36.mp4

Testing Instructions

• Try adding a heading block
• Press /. The slash inserter should appear.
• Further typing should narrow down block selections. Pressing enter or an option should replace the block entirely with the chosen block.
• Repeat testing for the list and quote blocks.
• Verify that these transformations work in the Post, Site and Widget editors.

[carolinan]

Love this! It is working well in my test.

[gwwar]

Thanks for the review @carolinan! I'll try landing this and we can see if folks had any feedback. This one is pretty easy to turn off again if we need to do so.