feature(is-registered): added a boolean method to indicate if a provider type or name is registered.

[gbtb16]

Description

• It was not possible to check whether a type T of instance or builder was registered in the kiwi global or scoped providers. This limited the ability to register instances only if they were not registered.

What has changed?

• It is now possible to check if a provider is registered by its type and/or name.
  • Since previously there couldn't be two instances with the same registered type unless the second one had a name, it's safe to create this method that uses what was in _setProvider.

• As mentioned in the comment of method, if an instance is defined with a name and you use the isRegistered method and do not pass its name, false will be returned if no instance was created with a name.

• Since what was checked within _setProvider was reused, it was changed to use the isRegistered method to avoid unnecessary code repetition.

• All 21 tests are passed correctly.