Fix bad FunctionRegistry#remove logic

[Efnilite]

Problem

Bug explanation
When removing functions from the FunctionRegistry, the remove method would attempt to get the namespace of the function being removed. To do this, it uses the script of the signature of the function. Functions registered in Java have no script, but functions registered in Skript do.

Therefore, if a function was previously registered and unregistered, it would leave an empty namespace in the namespaces map. The old remove logic would then incorrectly assume that this is the namespace of the function being removed, since the Skript-based function has a non-null script, and then not do anything.

Since all unit tests are made using Java, remove would work fine for Java functions, as the namespace is always null.

Scope enforcement
Also changes the register methods to enforce specifying exactly which namespace to register in, which increases clarity in where functions actually end up. Therefore, you are no longer able to register a local function to the global namespace by setting null as the namespace, and you are no longer able to register a global function to a local namespace by specifying a namespace in the Registry#register(String namespace, String name, Class<?> args...) arguments.

Solution

Update the code to check whether the signature is local to determine which namespace to use.

Testing Completed

Added a unit test which recreates the exact scenario in which this error would occur.

Supporting Information

No breaking changes as FunctionRegistry is package-protected.

---

Completes: none
Related: none