Add math.isfinite() utility function

[yl-whosit]

Very simple utility function present in many languages working with floats.

math.isfinite(n) returns true if n is not an infinite number (math.huge or -math.huge) and not a NaN.

Can probably be implemented using standard C++ function:
https://en.cppreference.com/w/c/numeric/math/isfinite
not sure if that would be better, or this is good enough.

UPD: (please squash this if you decide to merge :p)

[appgurueu]

i think this is useful especially given that finite numbers are something the engine frequently requires.

(btw: implementing this in C++ would be more complex and most certainly less efficient given LuaJIT.)