feat: localize subscription event widget

[kevmo314]

Introduces localization support for Twitch subscription events in the rtchat app.

• Localization Implementation: Adds AppLocalizations calls in subscription_event.dart to replace hardcoded strings with localized versions. This includes the subscription event, subscription gift event, and subscription message event widgets.
• Style Tags Preservation: Ensures that style tags are maintained within the localized strings to keep the text styling consistent post-localization.
• Localization Strings Addition: Updates app_en.arb with new entries for the localized strings used in subscription_event.dart, including placeholders for dynamic content such as subscriber names, tier levels, and cumulative totals.

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