This is the development part of the project. To create a working extension you need to put the repository content folder in a new directory and create an "extension" folder next to it. Then put the manifest.json file from the repository there. yarn build will place the finished extension files in this folder, which can be used in the browser.
final state of folders:
📂 extension
📄 manifest.json
📂 repository-folder
📂 pages
📂 public
📂 styles
📄 package.json