Fix tmux vim navigator movement

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.

tmux/.config/sh/alias.d/tmux.sh

@@ -2,6 +2,10 @@
 
 exist() { type "$1" >/dev/null 2>&1; }
 
+# alias tmux to follow xdg-specification
+# shellcheck disable=2139
+alias tmux="tmux -f ${XDG_CONFIG_HOME:-$HOME/.config}/tmux/tmux.conf"
+
 # show a list of running tmux sessions
 alias tl='tmux list-sessions -F "#{session_name}" 2>/dev/null'