add typings to package.json

Author: developit

package.json

@@ -22,6 +22,7 @@
     "prepublish": "npm run -s build",
     "release": "npm run build -s && git commit -am $npm_package_version && git tag $npm_package_version && git push --follow-tags && npm publish"
   },
+  "typings": "src/index.d.ts",
   "repository": "developit/linkstate",
   "keywords": [
     "preact",