Improve scope system

[ben-sb]

The current approach to gathering variable declarations and references is too loose and can result in incorrect deobfuscation. Since Shift doesn't provide this for us (like Babel does for example), a proper scope system should be implemented to collect all declarations and references.

Requirements:

• Correctly handle all declarations and references
  • Including modern features such as object and array destructuring, import and export etc
• Allow transformations to subscribe to the scope system, and request for the declarations and references to be updated
  • Could alternatively handle scopes once, then have each transformation register any changes it has made
    • Downside of this is writing transformations will become tedious

Can use a similar approach to Babel's scope system where we collect all declarations and references in one pass, then link them afterwards.
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