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Also sort and remove duplicates, and pipe to fmt to improve output format.

Fixes #8755

The statements for selecting a random theme in oh-my-zsh.sh and the themes
plugin are duplicate. Most people eventually settle on a theme, making those
lines in oh-my-zsh.sh superfluous. To address those, it may makes sense to put
the random theme functionality into a theme of its own (since themes are just
zsh scripts.

`lstheme` command used to list only the themes listed in `$ZSH/themes/` directory. This commit adds themes in `$ZSH_CUSTOM/themes/` also to the theme listings.