Improve GDScript sorting mechanisms for Arrays of custom objects.

[TestSubject06]

We have a sort_custom in Array that allows us to sort arrays based on a callable, but because of the way it's implemented (quite necessarily) it causes a GDScript / C++ barrier crossing every time the callable is invoked. Even relatively small arrays (300-400 entries) take significant time to sort, where a very large amount of the time isn't spent sorting, but is instead spent crossing the language boundary.

It would be nice if there was a way we could point at a specific field in the object, even if there were plenty of restrictions applied on the data types allowed to be used. Something like an Array.sort_by("property").

A thing I have to do a lot of in my projects is to sort a list of things by their distance to something of interest. Right now the fastest way to do that is to precalculate the distance for each thing, store that in a Dictionary[float, Node], and then using .sort() on that dictionary, which is not very intuitive - I only stumbled onto the sort method by accident. It just feels like that's not how that's supposed to be used.

Alternatively it would be similarly useful to be able to reserve a property on custom objects where we can put a value that would be used by regular sort() calls, similar to how you can override the comparison operators in other languages - but without needing to open the whole can of worms that hidden control flow opens up. Imagine having a sort_value property or something similar that you can set before calling sort() so that you don't have to cross the boundary so many times.