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_integer.wren

@@ -0,0 +1,11 @@
+System.print(123.isInteger) // expect: true
+System.print(123.0.isInteger) // expect: true
+System.print(0.isInteger) // expect: true
+System.print(1.0000001.isInteger) // expect: false
+System.print((-2.3).isInteger) // expect: false
+
+// NaN is not an integer.
+System.print((0/0).isInteger) // expect: false
+
+// Infinity is not an integer.
+System.print((1/0).isInteger)  // expect: false