Run mvn package in the "language-server" folder to generate language-server/target/language-server.jar
Run npm run compile to generate out/main/extension.js and out/web/extension.js
Run vcse package to generate the .vsix
To test the .vsix you can use a separate instance of Visual Studio Code Insiders. You can either drag and drop the .vsix file in the extensions view or use the Install from VSIX... command.