Add VariantArray duplicate method for deep copy

[mivort]

According to Godot's Array docs, duplicate method can accept deep boolean parameter to perform a deep array copy. This patch adds this functionality to VariantArray's interface. Simple deep copy functionality test was also added to test_array section.

[ghost]

Looks good to me. Thanks!

bors r+

[bors]

Build succeeded:

• ci

[Bromeon]

Quick question regarding method names, we have

• Variant::clone()
• GodotString::clone()
• Dictionary::duplicate()
• VariantArray::duplicate()
• VariantArray::duplicate_deep()

And for ref-increment we use new_ref(), if I'm not mistaken.

From a user perspective, maybe we should highlight this somewhere in the documentation?

Is there a semantic difference between clone and duplicate[_deep] or is the idea "when it supports both shallow and deep copy, use duplicate/duplicate_deep, otherwise use clone?

[ghost]

@Bromeon I'm aware that it's a bit messy right now. For clone vs duplicate, my understanding is that Clone on a reference-counted type should be a reference increment (i.e. not even a shallow copy), to be consistent with standard types like Rc, Arc, even though the reference-counting is "internal" in the case of variant collections. That does make NewRef seem redundant, and I think that would warrant a separate issue for discussion.