fix(ui_localizations): loading Traditional Chinese

[LaemonT]

Description

I found that the firebase_ui_localizations package includes a translation for Taiwan (ZhTWLocalizations), but has not been used appropriately. So I follow the same approach of how the flutter_localizations package gets the appropriate translation and how to fallback when the scriptCode or countryCode is not defined.

Related Issues

Closes #33

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[lesnitsky]

@gary6lin Thanks for submitting a PR! Could you please add unit tests verifying that traditional Chinese loads correctly now? Also might be good to add a test verifying that localization overrides still work. Unit test under firebase_ui_localizations package should be fine.

[lesnitsky]

@gary6lin could you please run melos run format to make code_health/format green.