Ensure Position's and Range's immutability

[Reinmar]

Strongly related to https://github.com/ckeditor/ckeditor5-engine/issues/897

Currently, Position#offset has a setter and the property itself is not defined as @readonly. What's worse, we use that setter in some places.

As we understood when working on https://github.com/ckeditor/ckeditor5-engine/issues/897 we need mutable positions and perhaps ranges too, but they should be used in a controlled environment only (inside computational heavy algorithms). Public API should not use them. And publicly, Position and Range should be immutable.

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[CKEditorBot]

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