Add tmux config and replace zellij with tmux in zshrc.

Zhrc also has some light refactoring, and has added a zsh local file
which gets sourced if it exists, useful for adding local environment
variables

home/.config/tmux/tmux-set-resize-mode.sh

@@ -0,0 +1,13 @@
+#!/bin/zsh
+
+desired_state=$1
+current_state=$(tmux show-environment -g @resize_mode 2>/dev/null | cut -d '=' -f 2)
+
+[ "$current_state" = "$desired_state" ] && exit 0
+
+tmux set -g @tmux_resize_mode $desired_state
+if [ "$desired_state" = "1" ]; then
+    tmux display-message "Resize mode ON"
+else
+    tmux display-message "Resize mode OFF"
+fi