tmux: add support for forced unicode support (#5166)
tmux: add support for forced unicode support (#5166)

When ZSH_TMUX_UNICODE=true then tmux is informed that UTF-8 is supported.

Closes #5166

monological/oh-my-zsh,monological/oh-my-zsh,monological/oh-my-zsh,monological/oh-my-zsh