script: Clean up attribute access a little bit in Element (#43064)

- Use modern Rust conveniences such as `unwrap_or_default`
 - Unabbreviate `attr`
- Unify the lowercase ASCII name assertion and make it a debug assertion
 - Use `unreachable!` instead of panic
- Expose two attribute getters that follow the behavior of two
specification concepts and what we expect internally in Servo:
- One that takes a namespace, but does not require lowercase attribute
names. ([specification
concept](https://dom.spec.whatwg.org/#concept-element-attributes-get-by-namespace>))
- One that does not take a namespace, but does require lowercase
attribute names. ([specification
concept](https://dom.spec.whatwg.org/#concept-element-attributes-get-by-name))
Testing: This should not really change behavior so should be covered by
existing tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/html/htmlhyperlinkelementutils.rs

@@ -2,7 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
-use html5ever::{local_name, ns};
+use html5ever::local_name;
 use servo_url::ServoUrl;
 
 use crate::dom::bindings::cell::DomRefCell;
@@ -185,10 +185,7 @@ impl<T: HyperlinkElement + DerivedFrom<Element> + Castable + NodeTraits> Hyperli
         // Step 2. Let url be this's url.
         USVString(match *self.get_url().borrow() {
             None => {
-                match self
-                    .upcast::<Element>()
-                    .get_attribute(&ns!(), &local_name!("href"))
-                {
+                match self.upcast::<Element>().get_attribute(&local_name!("href")) {
                     // Step 3. If url is null and this has no href content attribute, return the
                     // empty string.
                     None => String::new(),
@@ -428,9 +425,7 @@ impl<T: HyperlinkElement + DerivedFrom<Element> + Castable + NodeTraits> Hyperli
         // Step 1. Set this element's url to null.
         *self.get_url().borrow_mut() = None;
 
-        let attribute = self
-            .upcast::<Element>()
-            .get_attribute(&ns!(), &local_name!("href"));
+        let attribute = self.upcast::<Element>().get_attribute(&local_name!("href"));
 
         // Step 2. If this element's href content attribute is absent, then return.
         let Some(attribute) = attribute else {