Migrate nord.tmux file to ZSH shell

nord.tmux

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+#!/usr/bin/env zsh
 # Copyright (C) 2017-present Arctic Ice Studio <[email protected]>
 # Copyright (C) 2017-present Sven Greb <[email protected]>
 
@@ -7,14 +7,13 @@
 # License:    MIT
 # References:
 #   https://tmux.github.io
-
 NORD_TMUX_COLOR_THEME_FILE=src/nord.conf
 NORD_TMUX_VERSION=0.3.0
 NORD_TMUX_STATUS_CONTENT_FILE="src/nord-status-content.conf"
 NORD_TMUX_STATUS_CONTENT_NO_PATCHED_FONT_FILE="src/nord-status-content-no-patched-font.conf"
 NORD_TMUX_STATUS_CONTENT_OPTION="@nord_tmux_show_status_content"
 NORD_TMUX_NO_PATCHED_FONT_OPTION="@nord_tmux_no_patched_font"
-_current_dir="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )"
+_current_dir="${0:a:h}"
 
 __cleanup() {
   unset -v NORD_TMUX_COLOR_THEME_FILE NORD_TMUX_VERSION