parse.y: Traverse AST post parsing to detect break/next usage issues

Seemed easiest to do it via the traversal infrastructure as we do not
otherwise track enough context/scope when instantiating break or next
statements.

Might be worth moving this out of src/parse.y, but didn't exactly know
where. Or maybe we wait until there's more such trivial validations
popping up

Fixes #2440

src/ScriptValidation.h

@@ -0,0 +1,12 @@
+// See the file "COPYING" in the main distribution directory for copyright.
+#pragma once
+
+namespace zeek::detail
+	{
+
+/**
+ * Run extra validations on the parsed AST after everything is initialized
+ * and report any errors via zeek::reporter->Error().
+ */
+void script_validation();
+	}