feat: add compile-time error for 'super' used outside method context

Add isInsideMethod helper that walks the compiler's parent chain to detect
if the current compilation scope is within a method. Use it in super_()
to emit a compile error when 'super' is used at top level or inside a
top-level function, with corresponding test cases for both scenarios.

test/super/super_at_top_level.wren

@@ -0,0 +1 @@
+super.foo // expect error

test/super/super_in_top_level_function.wren

@@ -0,0 +1,3 @@
+fn {
+  super.foo // expect error
+}