Specify types in package.json

Using this package in a Typescript project currently requires adding a
module declaration because the package does not provide any types.

This should fix that.

Author: sambostock

package.json

@@ -3,6 +3,7 @@
   "version": "1.0.2",
   "description": "Validator module powered by validate-with-xmllint",
   "main": "build/index.js",
+  "types": "build/index.d.ts",
   "scripts": {
     "build": "yarn run clean && yarn run cp-schemas && tsc",
     "cp-schemas": "mkdir -p build/schemas && cp -R schemas build",