move 'serve' command logic into a submodule of 'cli'

The changes to 'main.rs' and 'cli/serve.rs' in this commit are almost
pure code-motion.

src/cli.rs

@@ -1,9 +1,16 @@
 //! Integration with `clap`
+//!
+//! CLI argument structs are defined in this module. Execution logic for each
+//! command goes in a submodule.
 
 use std::path::PathBuf;
 
 use clap::{Parser, Subcommand};
 
+use crate::error;
+
+mod serve;
+
 /// Command line arguments
 #[derive(Parser)]
 #[clap(
@@ -49,3 +56,11 @@ pub(crate) struct ServeArgs {
     #[clap(flatten)]
     pub(crate) config: ConfigArg,
 }
+
+impl Args {
+    pub(crate) async fn run(self) -> Result<(), error::Main> {
+        match self.command {
+            Command::Serve(args) => serve::run(args).await,
+        }
+    }
+}