scripts: convert to ESM and deduplicate

This makes it a bit easier to add any new files that may need to be
cleaned up in the future.

Also allows us to use top-level await for the `yarn dev` task.

@@ -0,0 +13,4 @@
+	stderr: process.stderr,
+});
+
+execa('npx', ['gulp', 'watch'], {

@@ -0,0 +46,4 @@
+		});
+	} catch (e) {
+		await new Promise(resolve => setTimeout(resolve, 3000));
+		start();