Split existing GH Actions release workflow into ‘pre-release’ and ‘release’ workflows

[markcheret]

The two workflows should roughly work as follows:

Pre-release

1. fire on creation of a new release that is marked as pre-release;
2. build the Plugin based on the current state of main;
3. create an archive of that built Plugin and save it to the release as an artefact;
4. copy that archive to any remote staging environments we have set up.

Release

1. fire on creation of a new release that is marked as such:
   • also maybe possible to fire on-toggle ‘pre-release’ checkbox on an existing release to no?
2. build the Plugin based on the current state of main;
3. create an archive of that built Plugin and save it to the release as an artefact;
4. copy that archive to the SVN repo. trunk/ branch;
5. copy trunk/ to an appropriate tags/ folder and update the ‘Stable Tag’ field in trunk/readme.txt.