Migrating from vim-plug to Packer.nvim

Update log

• 2022-03-27: fix a typo in requiring lua modules
• 2022-02-08: change fastgit URL
• 2022-02-05: add new autocmd for auto-running command PackerCompile.

Over the past two years, I have been using vim-plug as my plugin manager for Nvim. Vim-plug is powerful, fast and amazing, especially considering that all its features are contained in a single vim script. It uses the old way to manage plugins, i.e., it will manage the loading of plugins directly.