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/break/closure_in_for.wren

@@ -0,0 +1,9 @@
+var f
+for (i in [1, 2, 3]) {
+  var j = 4
+  f = fn IO.print(i + j)
+  break
+}
+
+f.call
+// expect: 5