Include package.json file in ui-extension JAR

The ui-extension client library for Open UI will be released as an NPM package, but is also packaged as a JAR file so other modules can depend on a SNAPSHOT version of it.

Currently, the JAR only contains the transpiled code (ui-extension.min.js etc.). However, the JAR should also be usable as an NPM package, e.g. by unpacking it into node_modules. In that case the structure of the JAR should reflect the structure of the NPM package: it should include a package.json file, and the transpiled code should be located in a 'dist' sub-folder.