Add functions to pick a random element from an array

[timothyqiu]

Revival of #57007
Closes godotengine/godot-proposals#3454

In addition to the original functionality, I decided to also support

• Picking from packed arrays
  • To keep things simple, I made it a free function that takes any array type (Array, PackedStringArray, etc...) instead of a method on each array type. This also makes it easier to extend later if we want to support picking from more types.
  • To be consistent with functions like randi() and randi_range(), I named the function rand_pick().
• Using a random number generator
  • A similar rand_pick() method is added to RandomNumberGenerator.

# Regular array
var arr := [1, 2, 3, 4, 5]
print(rand_pick(arr))
print(rand_pick(arr))

# Packed array
print(rand_pick("Mon Tue Wed Thu Fri Sat Sun".split(" ")))

# With RandomNumberGenerator (useful for procedural generation)
var rng := RandomNumberGenerator.new()
rng.state = 2022
print(rng.rand_pick(arr))  # Prints 1
print(rng.rand_pick(arr))  # Prints 2
rng.state = 2022
print(rng.rand_pick(arr))  # Prints 1
print(rng.rand_pick(arr))  # Prints 2

[timothyqiu]

Superseded by #67444