Support underscores in numeric literals in GDScript

[RiverMesa]

A small, yet useful syntactic feature of at least a few other programming languages (Java, D, Crystal, probably others) is the ability to use underscores (_) in numeric literals (integers, floats etc.) for readiblity purposes - which I think would be a nice addition to GDScript, improving the readibility of some larger numbers, allowing for this

var some_large_number = 10_000_000

which is certainly a bit easier to read at a glance than

var some_large_number = 10000000

Which seems fairly in line with GDScript's design goal of being easy to read and learn.

(And while to me it mostly seems maybe-simple-enough to sneak into 3.0 still, I think it would be more appropriate to add in 3.1, which if I recall correctly a bunch of GDScript changes and improvements are planned? If this is desired, that is.)