Add package.json to exports

[Reeywhaar]

Missing package.json in "exports" creates warning in react-native. See uuidjs/uuid#444 for context

[jviide]

Hi! Thank you for this pull request. After applying these changes Node.js v19 throws the following error when trying to import Valita with import * as v from "@badrap/valita":

Error [ERR_INVALID_PACKAGE_CONFIG]: Invalid package config
/.../node_modules/@badrap/valita/package.json while importing file:///.../test.mjs.
"exports" cannot contain some keys starting with '.' and some not. The exports
object must either be an object of package subpath keys or an object of main
entry condition name keys only.

After looking a bit into the original issue with react-native, we tend to agree with D3.js's maintainer that this is best fixed in tooling side. In this case upgrading to react-native v0.69.4 or @react-native-community/cli v8.0.4 (or newer versions of them) should help.

However, thank you again for bringing this to our attention, we appreciate it 🙂

[jviide]

Oh, right, forgot to add: The Node.js error could be fixed by just putting the "node" and "default" keys under a "." key.

    "./package.json": "./package.json",
    ".": {
      "node": { ... },
      "default": ...
    }