ProviderBasedLocalizationService.Providers is not thread safe

Description

If a thread is adding a localization provider at the same time as a localized string is requested, this may lead to an InvalidOperationException due to the provider enumeration not being thread-safe.

Even though it is discouraged to call the LocalizationService during startup it is still possible, such as in the case of the constructor of the DisplayResolution class.

Breaking changes

• The Provider property is no longer supported, as it's not possible to expose a fully thread-safe IList implementation. Providers can instead be added and manipulated through methods AddProvider or InsertProvider. The current set of providers can be inspected by calling the ProviderList property.