Replace simple TreeWalker use with a more performance recursive loop

[Reinmar]

📝 Provide a description of the improvement

I've been doing some research and found out that using a TreeWalker is 50-100x slower than using a simple recursive loop.

In the previous iteration I noticed performance boosts when optimizing the model TreeWalker or position API used by it.

TreeWaker is quite heavy due to the number of checks that it performs or created position instances. Yet, from my experience, in a number of cases we need it to scan content of particular elements.

It'd be interesting to research what kind of (model and view) tree walker instances are created in terms of:

• ranges (entire element, from element start/end to given position, completely random),
• options (element end, characters, etc.),
• number of nodes inside (assuming that the content is quite long, the idea is to understand whether that particular walker is used for a small piece of content or a bigger fragment).

Based on this we could figure out whether we couldn't introduce and use some quick iteration methods for simpler cases:

• element.getDescendants()
• element.getDescendants( { from: descendantNode, to: 'end' } )
• etc

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