Remove model / view check from is() checks.

[jodator]

📝 Provide a description of the improvement

This was introduced by #4479. Right now I can see this cause performance lags as RootElement.is() is called quite often. This method uses string.replace which done hundred thousend times isn't good.

Another case is that I don't see any usage of this so 🤷‍♂️ why we have this. We can have another way of checking this (ie is it model position, element or whatever) - I'd suggest some flag or a helper on model. But I didn't research that.

📃 Other details

Below is a comparison of a "Performance: pasting data" test using "long (semantic)" input. Test consist invoking paste and doing undo.

Before (self times):

After  (self times):

And impact on whole code:

Before RootElement.is(): total time 9463 ms (includes super.is() calls)

After RootElement.is(): Total time: 3163ms (down by 3)

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[jodator]

@Reinmar I've changed some code to verify the approach and the speed up is visible. Now the question is do we want to keep model: & view: checks?

Changes are here: https://github.com/ckeditor/ckeditor5-engine/compare/i/6529?expand=1. Ps it is not PR ready code so minor work is needed there.

[jodator]

And the speedup comparison is here: #6528 (comment).

[Reinmar]

@Reinmar I've changed some code to verify the approach and the speed up is visible. Now the question is do we want to keep model: & view: checks?

IMO, yes. It was introduced later, that's why it's not used. But it should be. I had many cases myself when I had (for some reason) a view element passed in place of a model element or vice versa. If existing checks (especially in writers) were more precise, that would be caught ealier.

[Reinmar]

I reported #6538 and bumped #3818 higher as both would be a great improvement in our API.