feat: add log and pow methods to Num class with tests

Adds `log` method to Num class via DEF_NUM_FN macro, and `pow(_)` method via a new DEF_PRIMITIVE that calls C's `pow()`. Registers both primitives in wrenInitializeCore. Includes new test files `test/core/number/log.wren` and `test/core/number/pow.wren` covering basic cases and edge values (negative input for log, zero exponent for pow). Also adds Sven Bergström to AUTHORS.

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

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