fix(ts): action can be an async function (#1157)

Since we can use `parseAsync` for async functions so the action can be asynchronous.

related to #806

typings/commander-tests.ts

@@ -45,6 +45,14 @@ function increaseVerbosity(v: any, total: number) {
     return total + 1;
 }
 
+function syncCall() {
+    console.log("Sync success!");
+}
+
+async function asyncCall() {
+    return;
+}
+
 program
     .version('0.0.1')
     .usage('[options] <file ...>')
@@ -111,7 +119,11 @@ program
 
 program
     .command("name1", "description")
-    .command("name2", "description", { isDefault:true })
+    .command("name2", "description", { isDefault:true });
+
+program
+    .command("name3").action(syncCall)
+    .command("name4").action(asyncCall);
 
 program
     .exitOverride();

typings/index.d.ts

@@ -115,7 +115,7 @@ declare namespace commander {
      *
      * @returns Command for chaining
      */
-    action(fn: (...args: any[]) => void): Command;
+    action(fn: (...args: any[]) => void | Promise<void>): Command;
 
     /**
      * Define option with `flags`, `description` and optional