feat(document): add Document#getChanges

[AbdelrahmanHafez]

fixes #9096

[AbdelrahmanHafez]

Interesting that it would mark the build as failed even though all the checks in travis have succeeded
🤨

[vkarpov15]

Looks great 👍 the only suggestion I might have is naming.

What do you think of just Document#changes(), or Document#delta()?

[AbdelrahmanHafez]

I find naming functions/methods as verbs more descriptive, according to Bob Martin's Clean Code, so I'd prefer getChanges or getDelta over just changes or delta.

What do you think?

[vkarpov15]

Works for me, I think that's more consistent with the existing document API anyway. Document methods are verbs currently.