Refactoring HTMLOptionElement::Text into iterative style (#37167)

The original implementation of `HTMLOptionElement::Text` is recursive,
and the program may run out of stack space for a sufficiently large
number of iterations. The patch switches to an iterative implementation,
with `TreeIterator`.

Note that, instead of the usual `while let Some(node) = iterator.next()`
approach, we use `while let Some(node) = iterator.peek()` with the newly
added `TreeIterator::peek` function. This is because the choice of the
next node depends on some checks performed inside the `while` block,
whereas the `next` function determines the next node before entering the
block.

Moreover, the `TreeIterator::peek` function is added, instead of
wrapping the iterator into `Peekable`. This is because we will lose
access to the `TreeIterator::next_skipping_children` function if we wrap
it into `Peekable`.

Testing: This refactoring has to pass the existing tests.
Fixes: #36959

Signed-off-by: Kingsley Yung <kingsley@kkoyung.dev>

components/script/dom/node.rs

@@ -2035,6 +2035,10 @@ impl TreeIterator {
         self.current = None;
         Some(current)
     }
+
+    pub(crate) fn peek(&self) -> Option<&DomRoot<Node>> {
+        self.current.as_ref()
+    }
 }
 
 impl Iterator for TreeIterator {