refactor: replace Text.rich with StyledText in subscription events

[kevmo314]

Replaces Text.rich with StyledText in subscription_event.dart for enhanced text styling and consistency with follow_event.dart.

• Imports the styled_text package at the beginning of the file to enable the use of StyledText.
• Updates TwitchSubscriptionEventWidget, TwitchSubscriptionGiftEventWidget, and TwitchSubscriptionMessageEventWidget to use StyledText instead of Text.rich. This change allows for more flexible and consistent text styling across different event types.
• Utilizes StyledTextTag within StyledText to maintain existing text styles and apply additional styling where necessary, ensuring visual consistency with the rest of the application.

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