Change attribute serialization to explicit opt-in

[aduth]

Related: #865, #391

This pull request seeks to change attribute serialization behavior, currently inferred as the difference between a block's actual attributes at time of serialization and the attributes definition. With these changes, it will instead be required that a block explicitly control its comment encoding behavior by returning an object from an encodeAttributes function. This is implemented exactly as documented from the registerBlockType documentation.

Testing instructions:

Ensure that tests pass:

npm test

Verify that block comment serialization continues to behave as expected for Button align, Image align, and Quote style blocks, when changing these values and switching to Text mode.

[youknowriad]

don't you think the metadata( 'align' ) is better for consistency? It's just another way of parsing the attribute.

[aduth]

don't you think the metadata( 'align' ) is better for consistency? It's just another way of parsing the attribute.

I suppose it depends where we take attributes initialization. I tried to communicate my current state of mind earlier at #391 (comment) . If matching included encoded attributes, I'd agree we wouldn't need this function at all. I might try to spend some time tomorrow exploring AST selectors like the ESLint ones I mentioned in that comment. If those work out, I don't think we'd really want to use them for comment sourcing.

[aduth]

Closing in favor of #1905