set up structure for multiple cli commands

The previous cli is now behind the 'serve' subcommand.

src/cli.rs

@@ -0,0 +1,51 @@
+//! Integration with `clap`
+
+use std::path::PathBuf;
+
+use clap::{Parser, Subcommand};
+
+/// Command line arguments
+#[derive(Parser)]
+#[clap(
+    about,
+    version = crate::version(),
+)]
+pub(crate) struct Args {
+    #[clap(subcommand)]
+    pub(crate) command: Command,
+}
+
+#[derive(Subcommand)]
+pub(crate) enum Command {
+    /// Run the server.
+    Serve(ServeArgs),
+}
+
+/// Wrapper for the `--config` arg.
+///
+/// This exists to centralize the `mut_arg` code that sets the help value based
+/// on runtime information.
+#[derive(clap::Args)]
+#[clap(mut_arg("config", |x| {
+    let help = "Set the path to the configuration file";
+    x.help(help).long_help(format!(
+        "{}\n\nIf this option is specified, the provided value is used \
+         as-is.\n\nIf this option is not specified, then the XDG Base \
+         Directory Specification is followed, searching for the path `{}` \
+         in the configuration directories.
+        ",
+        help,
+        crate::config::DEFAULT_PATH.display(),
+    ))
+}))]
+pub(crate) struct ConfigArg {
+    /// Path to the configuration file
+    #[clap(long, short)]
+    pub(crate) config: Option<PathBuf>,
+}
+
+#[derive(clap::Args)]
+pub(crate) struct ServeArgs {
+    #[clap(flatten)]
+    pub(crate) config: ConfigArg,
+}