Add CI to help automated updating libsinguliar_julia_jll

[fingolfin]

Here's the idea: When a PR is merged that modifies files in deps/src, do this:

• get the current version of libsinguliar_julia_jll
• increment it (currently by incrementing minor and setting patch to 0
• create (or update) a branch oscar-system/libsinguliar (or so) on a fork of the Yggdrasil repo which updates the libsinguliar_julia_jll recipe to use the new master commit, and update to the new version)
• (force) push the branch
• if no PR for that branch already exists on Yggdrasil, open one (if one exists, the force push will have updated it)
• similarly, open or update a PR on Singular.jl with a corresponding change to Project.toml (of course it'll fail initially), and with a link to the Yggdrasil PR in its description

The result still would allow for accidentally making releases without up-to-date libsinguliar_julia_jll but it would make it quite a bit more convenient to do the right thing.