Fix type exports

Correctly exporting types in package.json. See issue and solution here microsoft/TypeScript#52363 for reference.

Author: rxw1

package.json

@@ -23,7 +23,8 @@
     "exports": {
     ".": {
       "require": "./dist/index.umd.cjs",
-      "import": "./dist/index.mjs"
+      "import": "./dist/index.mjs",
+      "types": "./dist/types/index.d.ts"
     },
     "./src/*": "./src/*"
   },