Add .zsh-theme to shellscript extensions

[itiut]

The .zsh-theme extension is used for oh-my-zsh theme files.

[mention-bot]

@itiut, thanks for your PR! By analyzing the history of the files in this pull request, we identified @gontadu, @egamma and @aeschli to be potential reviewers.

[msftclas]

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[aeschli]

Thanks @itiut !