`const` / `let` support for control flow flattening

Is your feature request related to a problem? Please describe. Control flow flattening doesn't include nodes with let or const declarations, which makes it useless with ES6 code

Describe the solution you'd like Add support (or configurable option) of let / const declarations to control flow flattening transformation.

Describe alternatives you've considered Additional transformation via babel-plugin that replaces let / const with var before obfuscation