RFC Implementation: Unify JSFunction and JSObject for consistent type system

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This PR implements the RFC to unify JSFunction and JSObject, addressing fundamental design issues in the inheritance hierarchy and aligning the type system with JavaScript's dynamic callability semantics.

Problem

The existing architecture had several critical issues:

1. Mismatch with JavaScript semantics: In JavaScript, any object can be callable through Proxy traps or other mechanisms, but our type system treated functions and objects as mutually exclusive
2. Complex inheritance hierarchy: JSFunction inheriting from JSObject created unnecessary complexity
3. Inconsistent closure hierarchy: JSClosure inherited from JSFunction while JSOneshotClosure inherited from JSObject
4. Type system gap: JSObject.global.Function.function!.new returns a JSObject that should be callable but wasn't in our type system

Solution

This implementation unifies the architecture while maintaining complete backward compatibility:

1. Unified Callability

All JSObject instances now support function call operations:

// Before: Only JSFunction could be called
let func: JSFunction = JSObject.global.alert.function!
func("Hello")

// After: JSObject is directly callable
let obj: JSObject = JSObject.global.alert
obj("Hello")  // Works directly

2. Consistent Inheritance

Both closure types now consistently inherit from JSObject:

// Before: Inconsistent hierarchy
JSClosure: JSFunction  // Inherited from JSFunction
JSOneshotClosure: JSObject  // Inherited from JSObject

// After: Unified hierarchy
JSClosure: JSObject  // Both inherit from JSObject
JSOneshotClosure: JSObject

3. Unified JSValue Representation

Functions are now represented as objects internally:

// JavaScript functions come in as .object(JSFunction) instead of .function(JSFunction)
// The .function property intelligently detects callable objects

Implementation Details

• Phase 1: Added complete callability functionality to JSObject, including callAsFunction, new, and all argument overloads
• Phase 2: Updated JSValue conversion to represent functions as objects while maintaining the .function property for backward compatibility
• Phase 3: Made JSClosure inherit from JSObject instead of JSFunction for consistency
• Added comprehensive documentation with migration guidance and deprecation notices

Backward Compatibility

All existing code continues to work unchanged:

• The .function property now intelligently checks both .function and .object cases
• JSFunction class remains available with full functionality
• All existing APIs maintain their behavior

Benefits

• Aligned with JavaScript semantics: Objects can be callable, matching JavaScript's dynamic nature
• Simplified architecture: Reduced inheritance complexity
• Consistent closure model: Both closure types follow the same pattern
• Future-ready: Prepared for JavaScript features like callable objects and Proxy traps
• Unified API surface: Developers can work with a single object model

Fixes #402.

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