Refactor closure capture logic and add closures test config

Refactors closure variable capture logic in compiler.ts for clarity and robustness, including more explicit local variable usage and conversion of capturedNames to a Set. Updates prescanNodeForFunctionExpressions to use an iterative approach to avoid stack overflows. Adds a 'closures' feature entry to tests/features.json for testing closure support.

Author: BlobMaster41