Add Random singleton

[Xrayez]

Closes godotengine/godot-proposals#1741.

Provides a global instance of RandomNumberGenerator available via scripting at global scope, as a complementary for the lack of additional RNG methods in GDScript functions.

Random.randomize()
Random.seed = hash("Goost")
var i = Random.randi() % 100
var f = Random.randf_range(-1.0, 1.0)

is equivalent to:

randomize()
seed(hash("Goost"))
var i = randi() % 100
var f = rand_range(-1.0, 1.0)

Exposing this class as a global instance allows to use methods such as randfn(), randi_range() available in RandomNumberGenerator, which are not available in GDScript.

A new local instance has to be created with:

var rng = Random.new_instance()

You can still use RandomNumberGenerator, but a new set of randomization methods may/should be implemented in Random class as well, such as godotengine/godot#43103, but before this can happen, godotengine/godot#43113 has to be backported to Godot 3.2.4.