PCH: generate headings in draft post

[danielabloch]

Is your feature request related to a problem?

Writing titles and headings for an article are both the last things I do and the last things I want to do. Can you help me come up with these based on what I've already written into the CMS?

Helping customers with generating headings is also a great step in helping them programmatically improve their SEO best practices.

See: H1 heading tags for SEO
See: H1-H6 headings and SEO

Describe the solution you'd like

Let me highlight a block of text and then show me a heading suggestion for that section.
Limit headings to 60 characters. If I need to make them longer I will.
If I don't like the first suggestion you offer, let me try it again.
I'd like to see all the headings you created next to each other so I can decide which one I want to use or modify.

Describe alternatives you've considered

The alternative right now would be letting customers use the Title Suggestions feature for this, but that's insufficient because the goals of a post title are different from the goals of a heading. The post title captures the entirety of the post and needs to generate interest in the article while also being informative. The heading is meant to organize the post and maintain a structured hierarchy of content sections.

Additional context

Related issue: #1886