feat: add ceil, cos, sin, sqrt, isNan native methods and division-by-zero tests

Implement five new Num methods (ceil, cos, sin, sqrt, isNan) in wren_core.c and register them on the Num class. Add corresponding test files for ceil, floor, is_nan, and sqrt. Extend divide.wren with division-by-zero edge cases (inf, -inf, nan). Remove stale TODO comments from minus, multiply, and plus tests.

test/number/is_nan.wren

@@ -0,0 +1,5 @@
+IO.print(1.isNan)      // expect: false
+IO.print((0/0).isNan)  // expect: true
+
+// Infinity is not NaN.
+IO.print((1/0).isNan)  // expect: false