refactor: pull things into own files, have a library

src/meta_commands.rs

@@ -0,0 +1,30 @@
+pub enum MetaCommand {
+    Exit,
+}
+
+pub enum MetaCommandParseError {
+    Unrecognized { cmd: String },
+}
+
+impl std::fmt::Display for MetaCommandParseError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            MetaCommandParseError::Unrecognized { cmd } => {
+                write!(f, "unrecognized meta-command {cmd:?}")
+            }
+        }
+    }
+}
+
+impl std::str::FromStr for MetaCommand {
+    type Err = MetaCommandParseError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.trim() {
+            ".exit" => Ok(MetaCommand::Exit),
+            cmd => Err(MetaCommandParseError::Unrecognized {
+                cmd: cmd.to_string(),
+            }),
+        }
+    }
+}