Add HashSet.duplicate()

[aaronfranke]

Follow-up to #106537 (comment)

HashSet has a copy constructor, but it's not obvious whether the constructor is truly copying the underlying data just from reading the usage. This makes the intent clearer, at the cost of adding a very small function to HashSet. There is already a similar API in Vector<> for duplicating those.

[Ivorforce]

I have some thoughts about this.
First off, I think this is generally a great idea.

What we have currently is the implicit copy constructor, which is very disingenuous about its real cost:

// Whoops, actually extremely expensive, because all strings are copied!
HashSet<String> set = p_hash_set;

It would already be much better to make copy constructors explicit only:

// Now it's somewhat clear we're making a new hash set.
HashSet<String> set = HashSet(p_hash_set);

However, I do prefer the .duplicate you suggest, even if it is a little non-C++ style.

Still, our priority for things like this needs to be consistency. If we're adding .duplicate, we need to remove the copy constructor. Otherwise, both APIs will be used, and it won't be clear which function is preferred.

Likewise, we can't just add .duplicate to one or two isolated container types. Otherwise, people will always have to guess if it's "one of those containers with .duplicate" or "one of those other containers with a copy constructor". We'd need to add them to all containers, and remove the copy constructors, in one fell swoop.

All of this makes this a bit bigger than a few innocuous lines, and it would need to be agreed on by the core team, in my opinion.

There is already a similar API in Vector<> for duplicating those.

That's a good point. I think the reason it exists for Vector is that it's exposed to GDScript (and it's probably in the codebase for consistency). It acts the same as the copy constructor from C++.
However, in GDScript it acts different from the copy constructor, because regular assignment references the original array, rather than making a CoW copy (like .duplicate). This creates a need for .duplicate, which likely prompted this function to be added in the first place.