Coming from #105 (review)

If I'm editing a text block and I add another, the "focus" switches to the newly inserted block, but the cursor stays in the previous block. I'm not sure if we're worrying about that at this point, but leaving a note here.

ToDo:

• Adds isSelected to RichText component and Paragraph, Heading blocks #237 handle focus on RichText-based blocks
• isSelected handling on PlainText-backed blocks #250 handle focus on other blocks

EDIT:
ToDo:
Recreating the ToDo list here after migration to FlatList

• Fix isSelected for RichText #270 handle focus on RichText-based blocks
• isSelected handling on PlainText-backed blocks #250 handle focus on PlainText-based blocks