Specify required fields for blocks

[runeh]

The spec says the following about blocks:

[..] arbitrary data objects in the array, only requiring _type-key.

and

In its simplest form it's an array of objects of a type with an array of children.

Only one of those statements is true presumably?

Additionally there are questions following from having the fields and requirements properly specified:

• What are legal names for _type on blocks
• What are reserved names for _type on blocks
• What are legal names for _type on spans
• What are reserved names for _type on spans
• What are the semantics of a block without a style if any?
• What are legal values for listItem?
• What are the semantics of a block without listItem but with level?
• And vice versa?
• Can items be nested, given that level is said to represent express visual nesting and hierarchical structures, not structure

And so on.