Gutenberg: Add the ability to set Markdown as the default block

There are many users who use Markdown a lot, and they'd benefit from being able to set it as the default block when they start a new post.

This has also been suggested and discussed at:

p1HpG7-5EW-p2 #comment-27865
p1HpG7-5EW-p2 #comment-27860
pafL3P-23-p2

Start by reviewing Gutenberg’s handling of the default block for new posts and the linked discussions. Determine how a user would choose Markdown as that default and how the behavior should be verified for newly created posts.

Written by the indexing model from the issue text.