Make building and testing sysext easier

[Areopagitics]

I just want to post my own journey of how I can make building, testing and contributing more accessible to others (maybe some of this could be added to the README eventually), because you often need to modify multiple cosmic crates to for building locally.

Please add any suggestions to hit the ground running for anyone wanting to contribute!

Here is a list in order of time saving importance:

1. Use Zed editor for debugging errors (save yourself the headache with VSCode freezing with even the lowest rust-analyzer settings i.e. 1 thread).
2. Merge updates using scripts/version-update.sh
3. Test sysext using VirtualBox with PopOS (easiest for sharing sysext folder and reloading new builds without breaking your native linux OS); build sysext natively on computer for speed (make sure to use mold). Here's a script for reloading the new sysext into /var/lib/extensions: sydo rsync -rv --chmod-D755,F755 --chown=$USER:$USER Desktop/cosmic/cosmic-sysext /var/lib/extensions && sudo reboot. (FUTURE IMPROVEMNT: Maybe we could mount new sysext directly qemu /var/lib/extensions as USER already)
4. Integrate other crates by using the commented out patch settings in the Cargo.toml. For example, if you made changes to one cosmic-settings-daemon, and you depend on it in cosmic-settings you need to uncomment and set in cosmic-settings Cargo.toml:

[patch.'https://github.com/pop-os/cosmic-settings-daemon']
cosmic-settings-daemon = { path = "../cosmic-settings-daemon" }
cosmic-settings-config = { path = "../cosmic-settings-daemon/config" }