Smoothen release workflow

[stickgrinder]

At the moment, Popochiu's development happens on a separate repository than the release one.

The reason is, the dev repo contains an example project, necessary to test the game engine locally during the development itself. We don't want that example game to be distributed with the engine though, so at the moment, @mapedorr pushes the engine changes to this repo by hand.

This has downsides, starting from @mapedorr having to put manual effort into every release, to permissions for new devs, to the fragmentation of issues refereces in commits, documentation in the wiki, etc.

The ideal workflow would involve a dependency manager, but that's not an option (and won't be soon)

A decent alternative may be:

• The development happens on this official repo, which will contain both the engine and an example project (Rename "Popochiu" example game #11 is mildly related)
• A script is available to create a release package, with only the necessary files for the add-on to work in Godot
• The script may be triggered by GitHub Actions, ideally adopting a git-driven approach, so that tagging main branch will result in automatic releases
• The same automation should ideally be able to publish on Godot's Assets Library

This issue can be split into smaller ones, like: once we have a script to perform releases, we can use it even without automation.
Going the extra mile with automatic GH releases and adding "on tag" autopublishing is a nice enhancement.

[stickgrinder]

PR open here: mapedorr/popochiu-dev#6