script: Reduce the use of unsafe in LayoutDom (#31979)

Remove the use of unsafe code in the layout wrappers of the DOM. The
main change here is that `unsafe_get()` no longer needs to be an unsafe
method, which allows us to transitively remove or reduce unsafe blocks
from callers. The function itself is not renamed, because it's still
a bit dangerous to start removing the layers of abstraction from actual
DOM nodes.

In addition `init_style_and_opaque_layout_data` can be merged into
`initialize_data`, which removes one more unsafe method.

Finally, a "Safety" section is added to some unsafe methods.

components/layout/wrapper.rs

@@ -140,7 +140,7 @@ where
         // We need the underlying node to potentially access the parent in the
         // case of text nodes. This is safe as long as we don't let the parent
         // escape and never access its descendants.
-        let mut node = unsafe { self.unsafe_get() };
+        let mut node = self.unsafe_get();
 
         // If this is a text node, use the parent element, since that's what
         // controls our style.