add element.hasProperties method

[ianstormtaylor]

Do you want to request a feature or report a bug?

Feature.

What's the current behavior?

It's common to want to check whether an element (inline/block) matches a specific set of properties. The naive way is to just check element.type === type, but that doesn't account for things like headings where you might have a data.level === 2 to check as well.

What's the expected behavior?

We should add a method that makes this easy to check. It should take Object|String, and use Block.createProperties to normalize them. And then check to see if the element has the properties in question.

block.hasProperties('quote')
inline.hasProperties({ type: 'inline', data: { url: 'https://example.com' }})

I think we should also add a getProperties that returns just the settable properties of the element (ie. type and data), omitting the other things that are present when fully serializing.

[ianstormtaylor]

Fixed by #3093.