feat: implement modulo operator for numbers with fmod and add test suite

Add num_mod primitive using fmod from math.h to support the % operator on number types. Register the new primitive in the numClass method table. Include a comprehensive test file covering positive, negative, and left-associative modulo cases, plus a divide-by-zero TODO comment in the existing division test.

test/number_divide.wren

@@ -2,3 +2,4 @@ io.write(8 / 2) // expect: 4
 
 // TODO(bob): Floating point numbers.
 // TODO(bob): Unsupported RHS types.
+// TODO(bob): Divide by zero.

test/number_mod.wren

@@ -0,0 +1,12 @@
+io.write(5 % 3) // expect: 2
+io.write(10 % 5) // expect: 0
+io.write(-4 % 3) // expect: -1
+io.write(4 % -3) // expect: 1
+io.write(-4 % -3) // expect: -1
+
+// Left associative.
+io.write(13 % 7 % 4) // expect: 2
+
+// TODO(bob): Floating point numbers.
+// TODO(bob): Unsupported RHS types.
+// TODO(bob): Error on mod by zero.