Undeprecate NamespacedKey(String, String) constructor or add new static factory method

[jpenilla]

Is your feature request related to a problem?

It's not always the case that an NSK will be owned by a plugin, for example with things added by datapacks. Currently you would need to use fromString and concatenate the key and value with : to avoid deprecated methods.

Describe the solution you'd like.

Undeprecate the constructor or add a new static factory method

Describe alternatives you've considered.

Using the deprecated method or fromString

Other

The javadoc should mention what the proper use case is and refer to the other factory methods/constructors for their purposes.