add stub for rewriter

Signed-off-by: Nikolaj Bjorner <nbjorner@microsoft.com>

src/ast/rewriter/finite_sets_rewriter.cpp

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+

src/ast/rewriter/finite_sets_rewriter.h

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+/*++
+Copyright (c) 2025 Microsoft Corporation
+
+Module Name:
+
+    finite_sets_rewriter.h
+
+Abstract:
+    Rewriting Simplification for finite sets
+
+
+Sampe rewrite rules:
+    set.union s set.empty -> s
+    set.intersect s set.empty -> set.empty
+    set.in x (set.singleton y) -> x = y
+
+Generally this module implements basic algebraic simplificaiton rules for finite sets
+where the signature is defined in finite_sets_decl_plugin.h.
+   
+--*/