LibJS: Report string length as the code point length, not byte length

For example, U+180E is 3 bytes, but should have a string length of 1.

Userland/Libraries/LibJS/Runtime/StringObject.cpp

@@ -5,6 +5,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Utf8View.h>
 #include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/PrimitiveString.h>
@@ -33,7 +34,7 @@ void StringObject::initialize(GlobalObject& global_object)
 {
     auto& vm = this->vm();
     Object::initialize(global_object);
-    define_direct_property(vm.names.length, Value(m_string.string().length()), 0);
+    define_direct_property(vm.names.length, Value(Utf8View(m_string.string()).length()), 0);
 }
 
 void StringObject::visit_edges(Cell::Visitor& visitor)

Userland/Libraries/LibJS/Tests/builtins/String/String.js

@@ -7,3 +7,10 @@ test("typeof", () => {
     expect(typeof String()).toBe("string");
     expect(typeof new String()).toBe("object");
 });
+
+test("length", () => {
+    expect(new String().length).toBe(0);
+    expect(new String("a").length).toBe(1);
+    expect(new String("\u180E").length).toBe(1);
+    expect(new String("\uDBFF\uDFFF").length).toBe(2);
+});