There's a closed pull request on the void-packages repository

zsh-syntax-highlighting: add .plugin.zsh
https://github.com/void-linux/void-packages/pull/28740

Description:
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#### General
- [ ] This is a new package and it conforms to the [quality requirements](https://github.com/void-linux/void-packages/blob/master/Manual.md#quality-requirements)

#### Have the results of the proposed changes been tested?
- [ ] I use the packages affected by the proposed changes on a regular basis and confirm this PR works for me
- [x] I generally don't use the affected packages but briefly tested this PR

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zsh-autosuggestions provides the zsh-autosuggestions.plugin.zsh file, this should provide its plugin file too.