Release branch management

[fendor]

We manoeuvred ourselves into a tough spot with our current branch management for releases and pre-releases. As it was initially not foreseen that we will have a long-running pre-release branch due to some issues with the cabal plugin, we did not split off a pre-release branch but tried to manage everything in master.

That tripped us up in the latest release as it makes it rather awkward to make releases for the current stable release of the vscode plugin.

Thus, I propose a new branch management policy:

• master is always the stable release
• pre-releases are performed from the branch release/<ver-num>.