Use one big config file

[zyxir]

Loading multiple files is pretty slow on Windows, partly due to the real-time scanning of Windows Defender. If all configuration files can be merged into one big file, it will speed up startup a bit. Additionally, the "one big file" could be furtherly split into two big files, one for built-in settings for quick editing, and one for third-party packages.

To better manage the one big file (or two big files), use outline-minor-mode，use-package and helper macros as much as possible.