Make vim XDG compliant

[ipwnponies]

We should move .vimrc and .vim/ files (also neovim's symlinks) to XDG_CONFIG_HOME.

How to configure vim: https://tlvince.com/vim-respect-xdg
neovim has existing support for checking the {xdg_config_dir}/nvim/init.vim

[ipwnponies]

To maintain support for vim (not XDG-compliant), we'll re-establish symlinks. This should be functionality noop, it's reversing the canonical path of vim files vs. neovim files.