Add append_array() method to Array class

[KoBeWi]

Closes godotengine/godot-proposals#1718

Vector class (used internally by Array) already had append_array method, so this PR pretty much only exposes it.

[akien-mga]

This is also consistent with Packed*Arrays which have append_array.

[akien-mga]

In the proposal @Zylann raises a good point about potentially missing the opportunity to optimize the += operator so that it doesn't translate to a = a + b but actually does the same as append_array internally:

godotengine/godot-proposals#1718 (comment)

This shouldn't prevent merging this for consistency with Packed*Array but we should likely discuss it first nevertheless (CC @vnen @reduz).

[vnen]

In the proposal @Zylann raises a good point about potentially missing the opportunity to optimize the += operator so that it doesn't translate to a = a + b but actually does the same as append_array internally:

That might be a bit tricky. For this to work well I think Variant should provide the "assign with operation" operator as well. From GDScript it's hard to tell if the code isn't typed.

[akien-mga]

Further comments from IRC:

13:22 <Akien> reduz: vnen: WDYT about #43398 ? I think it's fine to add the method since Packed*Arrays have it, but should we also do something to improve the `+=` operator to avoid doing a copy?
13:24 <vnen> Akien: oh, I just commented there
13:25 <vnen> I think we need the += operator in variant itself for this to work
13:33 <reduz> Akien, vnen: appending arrays is not really a high performance use case in general, so probably not worth it. I think if the function is exposed its better than having to implement += operators
13:36 <Akien> String::operator+=() does seem to mutate the original String though, so container-wise it's a bit inconsistent that Arrays can't do it.
13:36 <vnen> yeah, users should know better what to use in this case
13:36 <Akien> But yeah having the explicit method seems a good step already.
13:37 <vnen> if you use from GDScript it won't call the C++ operator, it will just create copy

So changing += wouldn't be that simple. Let's merge this as a good first step to let users optimize their allocations.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.2.4.