[alert] iconMapping should also support variant

Summary

The current iconMapping prop on the Alert only supports passing in severity.

A nested iconMapping prop could enable passing in both severity and variant.

Examples

...
iconMapping: {
        success: {
          filled: CheckmarkOutline,
          outlined: Checkmark,
          standard: Checkmark,
        },
        info: {
          filled: InfoOutline, 
          outlined: Info, 
          standard: Info,
        },
        warning: {
          filled: AlertOutline, 
          outlined: Alert, 
          standard: Alert,
        },
        error: {
          filled: ErrorOutline, 
          outlined: Error, 
          standard: Error,
        },
      },
...

Alert.js

<IconSlot {...iconSlotProps}>
      {icon || iconMapping[severity][variant] || iconMapping[severity] || defaultIconMapping[severity]}
</IconSlot>

Motivation

I'm trying to use only themeOverrides to fully theme the Alerts we have designed, including the icons.

Search keywords: alert, icon, iconMapping, variant