Fix proxy resolution, escodegen crash, and cache collision bugs

[erikbilyk]

Summary

This PR fixes several bugs encountered when deobfuscating large real-world scripts:

1. Cross-sandbox evalInVm cache collisions - Cache key now includes sandbox ID to prevent different sandbox contexts from sharing cached results
2. Cross-scope getDeclarationWithContext cache collisions - Removed src-based cache fallback that caused nodes with identical source but different scopes to share context
3. Proxy resolution correctness - Added target variable shadowing checks, target reference modification checks, self-replacement skip, and batch validation
4. escodegen crash on Null literals - Added missing value: null property to Null case in createNewNode, and added recursive validation of replacement node trees
5. Function constructor body parsing - Added function-body wrapper fallback so new Function("return this")() can be parsed
6. replaceIdentifierWithFixedValue - Support Identifier assignments alongside Literals, and only check conditional context for write references

Test plan

• Tested against multiple large obfuscated Roblox JavaScript files
• Verified proxy resolution produces correct values
• Verified no more escodegen TypeError crashes
• Verified Function constructor patterns are correctly unwrapped

[aliceofhartley]

Just fyi your PR fails 4 tests in the test suite. Also if you're doing work with large javascript files, you might be interested in my forks of restringer, flast, and obfuscation-detector. If you look through the branches I have a set of performance and memory optimizations.

I have PRs open for most of them, so hopefully they'll be merged at some point.