Before this commit, we had a yarn package set up in the root directory and also in `app/assets/javascripts`. That meant two `yarn install` calls and two `node_modules` directories. This commit merges them both into the root location, and updates references to node_modules.
A previous attempt can be found at https://github.com/discourse/discourse/pull/21172. This commit re-uses that script to merge the `yarn.lock` files.
Co-authored-by: Jarek Radosz <jradosz@gmail.com>
