fix(types): Export types for ESM.

fix #329.

Author: jwalton

package.json

@@ -9,7 +9,8 @@
         ".": {
             "import": "./dist/esm/index.js",
             "require": "./dist/cjs/index.js",
-            "default": "./dist/cjs/index.js"
+            "default": "./dist/cjs/index.js",
+            "types": "./dist/esm/index.d.ts"
         }
     },
     "files": [