List Block: Implement block switcher API

[youknowriad]

closes #704

Implements switching a list to text, heading or quote block.

[jasmussen]

Works great!

Are we sure we want Heading as a possible transformation here? I do realize this goes back to the discussion we had about converting text to a heading, and I'm okay not rehashing that — but it feels a little extra weird here, where only the first list item is converted to a heading.

I'm okay with either, we can always revisit later on if we find it weird, but I figured I'd ask the question.

[youknowriad]

Are we sure we want Heading as a possible transformation here?

Your choice.

[jasmussen]

Your choice.

💥

Let's remove it from the list for now. If you want to, you can convert to text first, then to heading. Thanks Riad!

[aduth]

Related: #689 (concatChildren proposed to be removed)

[youknowriad]

If anyone want to take a final look, I'll be merging this later.

[aduth]

Noting my reluctance to further build upon APIs we've identified as problematic, while also acknowledging that alternatives have yet to be fleshed out.

More at #689 (comment), #771

[youknowriad]

Yes, we'll have to reconsider these helper functions once we address #771