refactor: early return in fallback behavior

Fallback behavior should use early returns rather than else...if.
Using early returns instead makes it possible to include looping logic
without as many changes to the code.

For example, we can now add such a thing:

  for ( const provider of providers ) {
    const maybe_command = provider[cmd];
    if ( ! maybe_command ) continue;
    return maybe_command();
  }

Since the fact of whether or not this loop will find the command is
determined by the outcome, rather than a conditional expression, it
would be non-trivial to include it in the else...if chain.

src/executor.js

@@ -163,7 +163,9 @@ export async function execCommand(input) {
     // Handle help command
     const command = args[0];
     showHelp(command);
-  } else if (cmd.startsWith('!')) {
+    return;
+  }
+  if (cmd.startsWith('!')) {
     // Execute the command on the host machine
     const hostCommand = input.slice(1); // Remove the "!"
     exec(hostCommand, (error, stdout, stderr) => {
@@ -178,17 +180,20 @@ export async function execCommand(input) {
       console.log(stdout);
       console.log(chalk.green(`Press <Enter> to return.`));
     });
-  } else if (commands[cmd]) {
+    return;
+  }
+  if (commands[cmd]) {
     try {
       await commands[cmd](args);
     } catch (error) {
       console.error(chalk.red(`Error executing command: ${error.message}`));
     }
-  } else {
-    if (!['Y', 'N'].includes(cmd.toUpperCase()[0])) {
-      console.log(chalk.red(`Unknown command: ${cmd}`));
-      showHelp();
-    }
+    return;
+  }
+
+  if (!['Y', 'N'].includes(cmd.toUpperCase()[0])) {
+    console.log(chalk.red(`Unknown command: ${cmd}`));
+    showHelp();
   }
 }