feat: add natural logarithm method to Num class with C binding and test

Implement the `log` method on Num by adding a DEF_NUM_FN(log, log) macro invocation in wren_core.c, registering the corresponding primitive `num_log` on the numClass, and creating a new test file test/core/number/log.wren that validates positive and negative input behavior.

test/core/number/log.wren

@@ -0,0 +1,3 @@
+System.print(3.log)        // expect: 1.0986122886681
+System.print(100.log)      // expect: 4.6051701859881
+System.print((-1).log)     // expect: nan