Add field's merge support

[mikolamin]

Accordign to add merge field PR.
There’s a new MergeableFieldInterface, which field types must implement to opt into being mergeable.

[engram-design]

I've not looked into this yet, but is this a completed solution and tested? I was under the impression that we'd need to provide migration handling as per Brandon's write-up?

[mikolamin]

In my case migration was successful with this changes, but during it I turned off comparing the field settings in the fields/merge script due to the fact that Hyper fields always have differern setting in the config file.

[engram-design]

Gotcha, I'll be looking into that as well as the same time as I tackle this. We're planning on cleaning up the field settings a bit with the field layouts.

[mikolamin]

Thanks, looking forward to it!