Ckeditor5: Change deleteContent() default behaviour to "merge"

[Reinmar]

As noticed in https://github.com/ckeditor/ckeditor5-typing/issues/97 – all algorithms except the enter command do merge the blocks by default so it's more reasonable that this is the default behaviour.

I'm going to change the merge option to doNotMerge, so the default value can be false.

[Reinmar]

What do you think about leaveUnmerged? Will it be easier to read (with less visible double negation).