Bundle up all the files except the .git folder, LICENSE, and README.md as a zip file, rename it to .xpi, then import it to your browser. You may have to switch to something like Firefox Developer Edition so that it will let you edit about:config to disable xpinstall.signatures.required.
Or build and test with the web-ext utility.
https://extensionworkshop.com/documentation/develop/web-ext-command-reference/#web-ext-build
Don't forget to set your WakaTime API key in the add-on preferences or you'll have wasted your time installing this.
