(pkg/types): export typings and CJS builds

- typings now output for TS devs, who overlap a lot with MobX devs!

- ESM build now hidden behind `module`, while CJS dev and prod builds
  are under `main`
- CJS appears to still be necessary per #3, but not necessarily for
  browser support, for _test_ support it's also necessary
  - unless one is transpiling node_modules or using the `esm` package,
    Node's support for ESM is still behind a flag / not in LTS
    - also it may not support `module` field per docs? might need .mjs?

package.json

@@ -2,7 +2,9 @@
   "name": "mst-persist",
   "version": "0.0.2",
   "description": "Persist and hydrate MobX-state-tree stores",
-  "main": "dist/mst-persist.esm.js",
+  "main": "dist/index.js",
+  "module": "dist/mst-persist.esm.js",
+  "typings": "dist/index.d.ts",
   "files": [
     "dist/"
   ],