Add logseq-rot13

[admoss0]

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**Plugin Github repo URL: https://github.com/admoss0/ROT13

Github releases checklist

• a legal package.json file.
• a valid CI workflow build action for Github releases. (theme plugin for this).
• a release which includes a release zip pkg from a successful build.
• a clear README file, ideally with an image or gif showcase. (For more friendly to users, it is recommended to have English version description).
• a license in the LICENSE file.