fix(plugin-legacy): add index.d.ts at publish (#1457)

Author: DreierF

packages/plugin-legacy/package.json

@@ -4,7 +4,8 @@
   "license": "MIT",
   "author": "Evan You",
   "files": [
-    "index.js"
+    "index.js",
+    "index.d.ts"
   ],
   "main": "index.js",
   "types": "index.d.ts",