Fixes #225 - Implement libLLVMSharp custom functions

Changes:

• Added GetFunctionType() extension method to LLVMValueRef
• Added GetReturnType() extension method to LLVMValueRef
• Added comprehensive unit tests for both functions

Implementation:

• Uses existing LLVM APIs (LLVM.TypeOf and LLVM.GetReturnType)
• Follows established codebase patterns

Testing:

• Added 2 unit tests that verify functionality
• Tests check function type retrieval and return type extraction
• All tests follow existing test patterns

Usage:

var functionType = myFunction.GetFunctionType();
var returnType = myFunction.GetReturnType();