Prevent nodemon infinite loop on yarn dev after yarn clean

scripts/prepare-sources.js

@@ -2,3 +2,11 @@ const fs = require('fs');
 const path = require('path');
 
 fs.copyFileSync('package.json', path.join('src', 'package.json'));
+
+// These dir must exist for nodemon not to loop indefinitely.
+[
+    'build',
+    'public',
+].forEach(dir => {
+    if (!fs.existsSync(dir)) fs.mkdirSync(dir);
+});