highlight ^old^new pattern
highlight ^old^new pattern

by extending the already in place mechanism of checking for the first
histchar to check for the second as well

danielshahaf/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,stuha/wbr-zsh,jimmijj/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting,x-way/zsh-syntax-highlighting,zsh-users/zsh-syntax-highlighting,joshuarubin/dotfiles,m0vie/zsh-syntax-highlighting,joshuarubin/dotfiles,danielshahaf/zsh-syntax-highlighting-debian,codedecay/zsh-syntax-highlighting,ryexley/zsh-syntax-highlighting,fly19890211/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,phy1729/zsh-syntax-highlighting,g5pw/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,P71/zsh-syntax-highlighting,danielshahaf/zsh-syntax-highlighting-debian,joshuarubin/dotfiles,joshuarubin/dotfiles,zsh-users/zsh-syntax-highlighting,psprint/zsh-syntax-highlighting,m0vie/zsh-syntax-highlighting,zsh-readable/zsh-readable