feat: config crate with legacy env names

crates/config/src/errors.rs

@@ -0,0 +1,11 @@
+#[derive(Debug, thiserror::Error)]
+pub enum ConfigError {
+    #[error("configuration file {0} was named by CONFIG_FILE but does not exist")]
+    NamedFileMissing(String),
+    #[error("{0} is required but not set")]
+    Missing(&'static str),
+    #[error("invalid value for {variable}: {reason}")]
+    Invalid { variable: String, reason: String },
+    #[error("could not read configuration: {0}")]
+    Unreadable(#[from] Box<figment::Error>),
+}