AK: Replace the mutable String::replace API with an immutable version

This removes the awkward String::replace API which was the only String
API which mutated the String and replaces it with a new immutable
version that returns a new String with the replacements applied. This
also fixes a couple of UAFs that were caused by the use of this API.

As an optimization an equivalent StringView::replace API was also added
to remove an unnecessary String allocations in the format of:
`String { view }.replace(...);`

Userland/Applications/KeyboardSettings/main.cpp

@@ -83,8 +83,7 @@ int main(int argc, char** argv)
 
     while (iterator.has_next()) {
         auto name = iterator.next_path();
-        name.replace(".json", "");
-        character_map_files.append(name);
+        character_map_files.append(name.replace(".json", ""));
     }
     quick_sort(character_map_files);