Make de_net independent

[Indy2222]

A crate like de_net has the potential to be generally useful outside of Digital Extinction.

• Separate the messages part (the bincode::Encode enums) to a standalone crate since this part is DE specific
• Find a nice new name
• Extract the crate to a standalone repository (ideally with git subtree history)
• Setup CI, docs & basic infrastructure in the new repo
• Do a little bit advertising for the new crate
• Create yet another independent small crate which integrates networking crate with Bevy via a Plugin
• Use these new crates in Digital Extinction

[github-actions]

There is no activity on the issue for some time.