AK+LibWeb: Add {Fly,}String::to_ascii_{upper,lower}_case()

These don't have to worry about the input not being valid UTF-8 and
so can be infallible (and can even return self if no changes needed.)

We use this instead of Infra::to_ascii_{upper,lower}_case in LibWeb.

Userland/Libraries/LibWeb/HTML/DataTransfer.cpp

@@ -142,7 +142,7 @@ String DataTransfer::get_data(String const& format_argument) const
         return {};
 
     // 3. Let format be the first argument, converted to ASCII lowercase.
-    auto format = MUST(Infra::to_ascii_lowercase(format_argument));
+    auto format = format_argument.to_ascii_lowercase();
 
     // 4. Let convert-to-URL be false.
     [[maybe_unused]] bool convert_to_url = false;

Userland/Libraries/LibWeb/HTML/DataTransferItemList.cpp

@@ -76,7 +76,7 @@ WebIDL::ExceptionOr<JS::GCPtr<DataTransferItem>> DataTransferItemList::add(Strin
     // method's first argument.
     auto item = m_data_transfer->add_item({
         .kind = HTML::DragDataStoreItem::Kind::Text,
-        .type_string = MUST(Infra::to_ascii_lowercase(type)),
+        .type_string = type.to_ascii_lowercase(),
         .data = MUST(ByteBuffer::copy(data.bytes())),
         .file_name = {},
     });
@@ -100,7 +100,7 @@ JS::GCPtr<DataTransferItem> DataTransferItemList::add(JS::NonnullGCPtr<FileAPI::
     // converted to ASCII lowercase, and whose data is the same as the File's data.
     auto item = m_data_transfer->add_item({
         .kind = HTML::DragDataStoreItem::Kind::File,
-        .type_string = MUST(Infra::to_ascii_lowercase(file->type())),
+        .type_string = file->type().to_ascii_lowercase(),
         .data = MUST(ByteBuffer::copy(file->raw_bytes())),
         .file_name = file->name().to_byte_string(),
     });

Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -1506,7 +1506,7 @@ String HTMLInputElement::value_sanitization_algorithm(String const& value) const
         // https://html.spec.whatwg.org/multipage/input.html#color-state-(type=color):value-sanitization-algorithm
         // If the value of the element is a valid simple color, then set it to the value of the element converted to ASCII lowercase;
         if (is_valid_simple_color(value))
-            return MUST(Infra::to_ascii_lowercase(value));
+            return value.to_ascii_lowercase();
         // otherwise, set it to the string "#000000".
         return "#000000"_string;
     }

Userland/Libraries/LibWeb/HTML/HTMLLinkElement.cpp

@@ -137,7 +137,7 @@ void HTMLLinkElement::attribute_changed(FlyString const& name, Optional<String>
     if (name == HTML::AttributeNames::rel) {
         m_relationship = 0;
         // Keywords are always ASCII case-insensitive, and must be compared as such.
-        auto lowercased_value = MUST(Infra::to_ascii_lowercase(value.value_or(String {})));
+        auto lowercased_value = value.value_or(String {}).to_ascii_lowercase();
         // To determine which link types apply to a link, a, area, or form element,
         // the element's rel attribute must be split on ASCII whitespace.
         // The resulting tokens are the keywords for the link types that apply to that element.