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/while/truth.wren

@@ -0,0 +1,26 @@
+// False and null are false.
+while (false) {
+  IO.print("bad")
+  break
+}
+
+while (null) {
+  IO.print("bad")
+  break
+}
+
+// Everything else is true.
+while (true) {
+  IO.print("true") // expect: true
+  break
+}
+
+while (0) {
+  IO.print(0) // expect: 0
+  break
+}
+
+while ("") {
+  IO.print("string") // expect: string
+  break
+}