Improving lit

This project aims to modernize the LLVM Integrated Tester (lit) to improve its execution speed and long-term maintainability. Performance is currently hindered by synchronous subprocess management and the high OS-level overhead of launching separate processes for routine commands. To address this, the execution engine will transition to a non-blocking, asynchronous model using asyncio, and core built-ins like cat and diff will be moved in-process to eliminate redundant interpreter startups. Additionally, the project will resolve functional limitations in internal shell utilities and leverage Scalene for data-driven performance profiling. Deliverables include a modernized Python 3 codebase, an asynchronous execution engine, integrated in-process built-ins, and a comprehensive performance evaluation report.