Fixes microsoft#235889

Implement an iterative approach to manage the outline in the `OutlineModel` class.

* Add a stack to handle the children of the outline elements iteratively in the `_makeOutlineElement` method.
* Replace the recursive call with a loop that processes the stack in the `_makeOutlineElement` method.
* Add a stack to handle the children of the document symbols iteratively in the `_flattenDocumentSymbols` method.
* Replace the recursive call with a loop that processes the stack in the `_flattenDocumentSymbols` method.

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#235889

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#235889