Implement support for rich_text blocks

[parente]

The documentation for rich text elements is very limited. I used this block kit playpen to explore the schema of each element, and constraints on their children and attribute values.

• RichText block
• RichTextList element
• RichTextPreformatted element
• RichTextQuote element
• RichTextSection element
• Emoji object
• Text object
• Style object (name overlap with enum.Style, should naming deviate and use be TextStyle?)

[parente]

@imryche I've added the objects, elements, and block classes needed for rich text support in this pull request. It's already quite long so I stopped before diving into the generator aspects. I think those might be better in a second PR. What do you think?

[imryche]

@parente Thank you for the substantial work you've put into this. Yeah, the generator part is definitely could wait and is less critical at this stage.

[parente]

Thanks for creating and maintaining this library, @imryche.

I might try my hand at the generator PR as well. But as before, don't let me block you if you want to have at it.