feat: add pow(_) primitive to Num class for exponentiation

Implement the `num_pow` primitive that wraps C's `pow()` function, enabling
exponentiation on Num instances via the `pow(_)` method. Register the new
primitive in `wrenInitializeCore` and add a test file verifying basic cases:
2^4=16, 2^10=1024, and 1^0=1.

test/core/number/pow.wren

@@ -0,0 +1,4 @@
+System.print(2.pow(4))     // expect: 16
+System.print(2.pow(10))    // expect: 1024
+
+System.print(1.pow(0))     // expect: 1