Change default favicon size or allow to define favicon size.

### :speech_balloon:

Favicons are generated with a fixed size of 16px, which may not look optimal on Retina-like displays.

Is it possible to change the default favicon size? My recommendation is 32px.

For reference: https://github.com/fluttercommunity/flutter_launcher_icons/blob/b6e97ad48fbca0a560ff0111e1187aa9f5a2db65/lib/constants.dart#L40

Alternatively, this value could be made configurable via the configuration file.

Start by locating where the package generates the fixed 16px favicons and how its configuration file is parsed. Compare the two requested outcomes—using 32px by default or exposing the size as configuration—and verify the generated favicon size for the chosen behavior.

Written by the indexing model from the issue text.