chore: reintroduce lean.cpp in separate commit so Git doesn't freak out

src/shell/lean.cpp

@@ -0,0 +1,16 @@
+/*
+Copyright (c) 2021 Sebastian Ullrich. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+
+Author: Sebastian Ullrich
+*/
+
+// The actual main function is in `util/shell.cpp` and compiled into `libleanshared` to avoid issues with cross-lib C++.
+// It will eventually be replaced by Lean code anyway.
+// We put `main` in this separate file included only in `lean` so linking against `libleanshared` does not accidentally use it.
+
+extern "C" int lean_main(int argc, char ** argv);
+
+int main(int argc, char ** argv) {
+    return lean_main(argc, argv);
+}