Move building/publishing to CI

[phil294]

From #163:

PS: It would be great to have an automatic release process using GitHub actions (vs. you manually deploying it from local), to avoid this kind of problems and give more security and transparency on the deployed code in the marketplace. I think the one used for GitKraken might be a good starting point for inspiration.

The current release process includes several important manual steps such as testing, updating dependencies, and curating the changelog. I'll have a look but these are difficult to automate. However, I haven't thought of moving the actual bundling/publishing process into CI for transparency's and reproducibility's sake before. This is a good idea I think.