feat: add theme exporter for Zellij

[hougesen]

No description provided.

[olimorris]

Thanks for the PR! Really appreciated

[mmirus]

Thanks for this! Was just using one-half-dark previously. Took what the exporter generated and applied the vivid colors where different, since I'm using that variant in nvim:

// Colors - https://github.com/olimorris/onedarkpro.nvim
themes {
    onedarkpro-vivid {
        fg "#ABB2BF"
        bg "#282C34"
        black "#282C34"
        red "#EF596F"
        green "#89CA78"
        yellow "#E5C07B"
        blue "#61AFEF"
        magenta "#D55FDE"
        cyan "#2BBAC5"
        white "#ABB2BF"
        orange "#D19A66"
    }
}

// Choose the theme that is specified in the themes section.
// Default: default
//
theme "onedarkpro-vivid"