feat: add Num.round method for rounding numbers to nearest integer

Implement the `round` method on the Num class that rounds a number to the nearest integer using standard rounding rules (away from zero for .5). The implementation adds a new `num_round` primitive function in the VM core, registers it as a method on the Num class, includes documentation in the core module reference, and adds a comprehensive test file covering positive, negative, zero, and fractional cases.

test/core/number/round.wren

@@ -0,0 +1,8 @@
+System.print(123.round)      // expect: 123
+System.print((-123).round)   // expect: -123
+System.print(0.round)        // expect: 0
+System.print((-0).round)     // expect: -0
+System.print(0.123.round)    // expect: 0
+System.print(12.3.round)     // expect: 12
+System.print((-0.123).round) // expect: -0
+System.print((-12.3).round)  // expect: -12