feat: refactor and add ip saving

- clean up code to my new (and improved) standards
- print better miette diagnostigs for the error tests
- add an IP saving feature:
    save the last IP that successfully updated the records and usa that
    when restarting the service. This allows seamles upgrades of
    `webnsupdate` without having to manually trigger a DNS update

default.nix

@@ -19,6 +19,8 @@ let
           ".rs"
           # TOML files are often used to configure cargo based tools (e.g. .cargo/config.toml)
           ".toml"
+          # Snapshot tests
+          ".snap"
         ];
         isCargoLock = base == "Cargo.lock";
       in