Introduce `model.document.getRoots()`

[scofalik]

📝 Provide a description of the improvement

Introduce model.document.getRoots() that will be a shorthand to model.document.getRootNames().map( name => model.document.getRoot( name ) ).

After introducing multi-root editor, in many places we had to switch to iterating through all roots, instead of taking the default root. Because of that we have a ton of places like:

for ( const rootName of model.document.getRootNames() ) {
	const root = model.document.getRoot( rootName );
	
	// operate on `root`.
}

This doesn't look good.

At the same time, we could make model.document.roots a private property.

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