feat: add isInfinity and isInteger methods to Num class

Implements two new methods on the Num class: isInfinity checks if a number is positive or negative infinity using isinf(), and isInteger checks if a number has no fractional component, returning false for NaN and infinity. Includes C primitives, documentation, and test files.

test/core/number/is_infinity.wren

@@ -0,0 +1,3 @@
+System.print(123.isInfinity) // expect: false
+System.print((1/0).isInfinity)  // expect: true
+System.print((-10/0).isInfinity)  // expect: true