feat: treat null as falsey in conditional and logical operator branches

Update the VM interpreter to treat null as falsey alongside false in if, while, for, and logical AND/OR operations. Remove the TODO comment and adjust short-circuit logic in OP_JUMP_IF and OP_JUMP_IF_NOT branches. Refactor existing test files into subdirectories (if/, logical_operator/, while/) and add dedicated truthiness tests for each construct. Remove old test/if.wren and update and/or tests to exclude null/0/"" truthiness checks now that null is falsey.

test/logical_operator/and_truth.wren

@@ -0,0 +1,8 @@
+// False and null are false.
+IO.print(false && "bad") // expect: false
+IO.print(null && "bad") // expect: null
+
+// Everything else is true.
+IO.print(true && "ok") // expect: ok
+IO.print(0 && "ok") // expect: ok
+IO.print("" && "ok") // expect: ok