fix(v2): load plugin commands asynchronously

https://github.com/facebook/docusaurus/blame/master/packages/docusaurus/src/commands/external.ts#L12 changed the exported method to an async function, but the CLI loader was not updated to also run async, breaking things such as `npm run docusaurus docs:version <version>`.

packages/docusaurus/bin/docusaurus.js

@@ -252,12 +252,16 @@ function isInternalCommand(command) {
   ].includes(command);
 }
 
-if (!isInternalCommand(process.argv.slice(2)[0])) {
-  externalCommand(cli, path.resolve('.'));
-}
+async function run() {
+  if (!isInternalCommand(process.argv.slice(2)[0])) {
+    await externalCommand(cli, path.resolve('.'));
+  }
 
-cli.parse(process.argv);
+  cli.parse(process.argv);
 
-if (!process.argv.slice(2).length) {
-  cli.outputHelp();
+  if (!process.argv.slice(2).length) {
+    cli.outputHelp();
+  }
 }
+
+run()