feat(vercel): add vercel config for only deploying gh-pages branch on default

Author: andatoshiki

vercel.json

@@ -0,0 +1,8 @@
+{
+    "env": {
+        "BRANCH": "gh-pages"
+    },
+    "scripts": {
+        "vercel-deploy": "if [ \"$BRANCH\" != \"main\" ]; then echo \"Cannot deploy from $BRANCH branch\"; exit 1; fi"
+    }
+}