Use npm instead of yarn to build Theia

[tsmaeder]

What it does

This PR rewrites the Theia build process to use npm in stead of yarn. This required a couple of changes:

Contrary to yarn, npm does not invoke the prepare scripts of workspace packages in the order of their dependencies (npm/cli#3034). So instead of relying on yarn install to invoke the necessary build scripts, we have to invoke the build process explicitly after the npm install. The build process is built on this logic:

1. Each package has a script called "build". The goal of that target is to fully build all artifacts this repo produces. For regular Theia packages, this is just incrementally compiling the typescript sources of the package an it's dependencies, but for applications, for example, it means creating the application bundle. So calling "build" in an application package should be all that's needed in regular development
2. To build the repo initially after check-out, there are a couple of additinal steps necesary:
   • compile build tools (re-exports package)
   • generate re-exports where applicable
   • compile all packages
   • build alll application packages
     This can be achieved by invoking npm run build in the repo root.

What's missing:

• Doc is not up to date
• linting is currently broken

Fixes #13948

Contributed on behalf of STMicroelectronics

How to test

Make sure we can build Theia as described above and the apps work fine.

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines