This is a follow-up of request #14652

Now that all plugins have been harmonized, it is time to do the same for Tuleap Core.

"tuleap/src" should be the "entrypoint" for Tuleap Core. It should contain:

• package.json & package-lock.json. As long as it makes sense. Some dependencies might still be scoped per-application, as for kanban and planning-v2 in plugins
• jest.config.js
• tsconfig.json
• webpack.(common|dev|prod).js

This setup will make it easier for everyone to know where they should run "npm" commands. It also lets us unify themes and scripts builds by having only one command to build / watch both.

We will also take advantage of this big move to change a couple of things:

1. "Built" scripts should live in "tuleap/src/scripts". They should NOT live in "tuleap/src/www" as they are not meant to be loaded by themselves
2. "Loaded" standalone (one might say "legacy") scripts should stay in "tuleap/src/www". This layout makes it clear that everything in "www" is supposed to be loaded and is not handled by Webpack
3. "Core" assets should live in "tuleap/src/www/assets/core". Since they were the first assets, they were assigned to "tuleap/src/www/assets", but following request #14652, all plugins have their equivalent folder here. It's easier for the build script to manage (and delete) its own "core" folder. It's easier for devs who can rely on knowing "everything related to core is in the core folder".