fix: properly dispose loop-scoped variables and exit scopes on break

Replace the single `loopBody` index with a `Loop` struct that tracks scope depth, enabling the compiler to emit correct scope-exit code when a `break` statement is encountered inside nested blocks. Also add `WREN_DUMP_COMPILED_CODE` debug flag and new test cases for closures capturing loop variables after break, return inside loops, and nested loop scoping.

test/while/return_closure.wren

@@ -0,0 +1,10 @@
+var f = fn {
+  while (true) {
+    var i = "i"
+    return fn IO.print(i)
+  }
+}
+
+var g = f.call
+g.call
+// expect: i

test/while/return_inside.wren

@@ -0,0 +1,9 @@
+var f = fn {
+  while (true) {
+    var i = "i"
+    return i
+  }
+}
+
+IO.print(f.call)
+// expect: i