Fixes tmux xdg-compliance (and, more importantly, Tmux Plugin Manager's)
by setting the environment variable TMUX_PLUGIN_MANAGER_PATH to follow
xdg specifications. Tmux, due to not being xdg-compliant, needs to be
aliased to start with the `-f` option pointing into the configuration
directory.
Fixes tmux vim nagigator's controls being overwritten by other control
schemes in tmux.
The only file left in $HOME is .zshenv, which sets up zsh to source everything from XDG_CONFIG_HOME/zsh.
Shell files are split into sh and zsh directories, for global assignments (which should be posix compliant, work on any posix shell) like environemnt variables, xdg vars, and global aliases. zsh contains zsh specific customization (prompt customization, plugin loading, zsh completions).
Zsh initialization will pull from sh directory first, loading the respective mirror to its startup file (`.zprofile` loads `sh/profile` and `profile.d/*`, `.zshenv` loads `sh/env` and `sh/env.d/*` and `zsh/env.d/*`, `.zshrc` loads `sh/alias`, `sh/alias.d/*` and `zsh/alias.d/*`)
Once all is done, it will have loaded both global variables, aliases and settings, and zsh-only specifications. Other stow modules, if they want to add shell functionality, can include their aliases and functions in one of the above directories to automatically be picked up by zsh.
