Add build-browser-extension script

Created by: marekweb

Add build-browser-extension as a yarn script to simplify the process of building the browser extensions.

This is especially valuable when we submit the source code of the browser extension for review. The reviewers at Mozilla need to reproduce the build from source for in order to approve the browser extension. This makes it easier to run it and avoid mistakes such as running yarn or yarn build from the wrong directory.

With this change, it's now possible to run yarn run build-browser-extension from the root of the repository.