Add exports in to make /native work in Node 16.x ESM modules (#318)

If you're trying to import `react-content-loader/native` from Node 16.x
from within a ESM context (`"type": "module"`). You'll get the following
error:

```
node:internal/process/esm_loader:97
    internalBinding('errors').triggerUncaughtException(
                              ^

Error [ERR_UNSUPPORTED_DIR_IMPORT]: Directory import '~/node_modules/react-content-loader/native' is not supported resolving ES modules imported from /Users/swen/Code/cheetah/test/test.mjs
Did you mean to import react-content-loader/native/react-content-loader.native.cjs.js?
    at new NodeError (node:internal/errors:387:5)
    at finalizeResolution (node:internal/modules/esm/resolve:425:17)
    at moduleResolve (node:internal/modules/esm/resolve:1006:10)
    at defaultResolve (node:internal/modules/esm/resolve:1214:11)
    at nextResolve (node:internal/modules/esm/loader:165:28)
    at ESMLoader.resolve (node:internal/modules/esm/loader:844:30)
    at ESMLoader.getModuleJob (node:internal/modules/esm/loader:431:18)
    at ModuleWrap.<anonymous> (node:internal/modules/esm/module_job:76:40)
    at link (node:internal/modules/esm/module_job:75:36) {
  code: 'ERR_UNSUPPORTED_DIR_IMPORT',
  url: 'file:///~/node_modules/react-content-loader/native'
}
```

By setting up an explicit export for the `/native` subdirectory
the import works. No other changes are made to maintain backwards
compatibility.

package.json

@@ -12,6 +12,20 @@
   "module": "dist/react-content-loader.es.js",
   "jsnext:main": "dist/react-content-loader.es.js",
   "types": "dist/web/index.d.ts",
+  "exports": {
+    ".": {
+      "types": "./dist/web/index.d.ts",
+      "require": "dist/react-content-loader.cjs.js",
+      "import": "dist/react-content-loader.es.js",
+      "default": "dist/react-content-loader.cjs.js"
+    },
+    "./native": {
+      "types": "./native/native/index.d.ts",
+      "require": "./native/react-content-loader.native.cjs.js",
+      "import": "./native/react-content-loader.native.es.js",
+      "default": "./native/react-content-loader.native.cjs.js"
+    }
+  },
   "bugs": {
     "url": "https://github.com/danilowoz/react-content-loader/issues"
   },