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/script/dom/bindings/root.rs

@@ -757,7 +757,7 @@ where
 {
     /// Returns a reference to the interior of this JS object. The fact
     /// that this is unsafe is what necessitates the layout wrappers.
-    pub unsafe fn unsafe_get(self) -> &'dom T {
+    pub fn unsafe_get(self) -> &'dom T {
         assert_in_layout();
         self.value
     }