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

Implement the `round` method on the Num class in the Wren VM, which rounds a number to the nearest integer using standard rounding rules (away from zero for .5). The implementation adds a new `DEF_NUM_FN(round, round)` macro invocation in `wren_core.c`, registers the corresponding primitive `num_round`, and includes comprehensive documentation in the core module reference. A new test file `test/core/number/round.wren` validates behavior for positive, negative, zero, and fractional values. Also adds Kyle Charters to the AUTHORS file.

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