Add constants for minimum and maximum values for int and float data types

[blackears]

It can sometimes be useful to set a variable to the largest or smallest possible value. I'd like to see the following constants added to GDScript:

INT_MAX
INT_MIN
FLOAT_MAX
FLOAT_MIN

[AThousandShips]

Already tracked by #2411

[Calinou]

This is already tracked by #2411.