feat: Add light mode and dark mode options to registerSnackbarConfig

[Moseco]

I changed registerSnackbarConfig to allow the use of a general SnackbarConfig (like it is now) or the use of SnackbarConfig for light mode and dark mode.

showSnackbar will then use the appropriate SnackbarConfig based on what was supplied and whether the app is in light mode or dark mode.

This might be considered a breaking change because registerSnackbarConfig now only takes named arguments instead of a single positional argument like before.