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/if/dangling_else.wren

@@ -0,0 +1,3 @@
+// A dangling else binds to the right-most if.
+if (true) if (false) IO.print("bad") else IO.print("good") // expect: good
+if (false) if (true) IO.print("bad") else IO.print("bad")

test/if/else.wren

@@ -0,0 +1,10 @@
+// Evaluate the 'else' expression if the condition is false.
+if (true) IO.print("good") else IO.print("bad") // expect: good
+if (false) IO.print("bad") else IO.print("good") // expect: good
+
+// Allow block body.
+if (false) null else { IO.print("block") } // expect: block
+
+// Newline after "else".
+if (false) IO.print("bad") else
+IO.print("good") // expect: good

test/if/if.wren

@@ -0,0 +1,14 @@
+// Evaluate the 'then' expression if the condition is true.
+if (true) IO.print("good") // expect: good
+if (false) IO.print("bad")
+
+// Allow block body.
+if (true) { IO.print("block") } // expect: block
+
+// Assignment in if condition.
+var a = false
+if (a = true) IO.print(a) // expect: true
+
+// Newline after "if".
+if
+(true) IO.print("good") // expect: good

test/if/truth.wren

@@ -0,0 +1,32 @@
+class Iter {
+  new(value) { _value = value }
+  iterate(iterator) { return _value }
+  iteratorValue(iterator) { return "value" }
+}
+
+// False and null are false.
+for (n in new Iter(false)) {
+  IO.print("bad")
+  break
+}
+
+for (n in new Iter(null)) {
+  IO.print("bad")
+  break
+}
+
+// Everything else is true.
+for (n in new Iter(true)) {
+  IO.print("true") // expect: true
+  break
+}
+
+for (n in new Iter(0)) {
+  IO.print(0) // expect: 0
+  break
+}
+
+for (n in new Iter("")) {
+  IO.print("string") // expect: string
+  break
+}