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/for/syntax.wren

@@ -0,0 +1,23 @@
+// Single-expression body.
+for (i in [1, 2, 3]) IO.print(i)
+// expect: 1
+// expect: 2
+// expect: 3
+
+// Block body.
+for (i in [1, 2, 3]) {
+  IO.print(i)
+}
+// expect: 1
+// expect: 2
+// expect: 3
+
+// Newline after "for".
+for
+(i in [1, 2, 3]) IO.print(i)
+// expect: 1
+// expect: 2
+// expect: 3
+
+// TODO: Return from inside for loop.
+// TODO: Return closure from inside for loop.