Add nextElementSibling and previousElementSibling polyfills

[NickColley]

Raising this to see if I'm on the right track.

Do we need to polyfill CharacterData?

So it looks like this feature is used in the node type of NonDocumentTypeChildNode since it can be Element and CharacterData. Does this block this improvement, or can we continue and consider a wider NonDocumentTypeChildNode polyfill later that would also sort CharacterData?

• NonDocumentTypeChildNode.nextElementSibling
  

Closes #338

[claassistantio]

All committers have signed the CLA.

[NickColley]

@JakeChampion think this is ready for another review, I believe the tests have not run since I'm an external contributor and don't have access to secure credentials?

I've also left a question about CharacterData in the main pull request description I'm not sure about.

[JakeChampion]

Tests are now running on CircleCI :-) Thank you for the pull request!

[JakeChampion]

Do we need to polyfill CharacterData?

So it looks like this feature is used in the node type of NonDocumentTypeChildNode since it can be Element and CharacterData. Does this block this improvement, or can we continue and consider a wider NonDocumentTypeChildNode polyfill later that would also sort CharacterData?

A wider NonDocumentTypeChildNode can be added later 👍

[NickColley]

Thanks for sorting those issues for me 👍

[JakeChampion]

@NickColley Thank you so much for contributing these polyfills! They will be in the next release :-)