Introduce GitHub CI for building the package to upload the stores

[karakanb]

So far I have been manually:

• checking out the master branch
• changing the version number in the manifest.json file
• tagging the release
• building the project
• zipping the source code

all manually. This process is cumbersome, and it causes me to make mistakes, as well as producing releases slowly. Ideally, I'd like to be able to publish to the stores automatically as soon as a release is built, but that might be tricky due to the lack of tools out there currently to publish to the stores via an API; however, there's no reason to not to have a simple CI pipeline that'll simplify this process.

Create a new release

Every time there's a new release, we'd like to:

• update the manifest.json with the new tag
• build the source code and make the ZIP archive available
• create a ZIP archive of the source code, excluding the dependencies

This way, I can simply download the files and upload them to the stores as a first step.