Enhance logging system with levels and timestamps

- Add new logging functions: nvm_log, nvm_debug, nvm_info, nvm_warn, nvm_error
- Implement log levels (DEBUG, INFO, WARN, ERROR)
- Add timestamps to log messages
- Create nvm_run_command for logging command executions
- Implement nvm_time_operation for timing long-running operations
- Add nvm_log_env to log environment information
- Update nvm function to log version and environment on start