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;