Add modularized imports to the exports field
I was trying to make `@phosphor-icons/react` work with the new [modularized imports](https://nextjs.org/blog/next-13-1#import-resolution-for-smaller-bundles) feature in Next.js. 

It should:
1. Improve the bundle size
2. Increase compilation speed thanks to skipping unused files.

I tried adding this configuration to my `next.config.js`:
```js

const config = {
// ...
  modularizeImports: {
    '@phosphor-icons/react': {
      transform: '@phosphor-icons/react/dist/icons/{{member}}',
    },
  },
// ... 
}
```

Unfortunately, it fails with the following error:
```
Error: Package subpath './Trash' is not defined by "exports" in /Users/[REDACTED]/node_modules/@phosphor-icons/react/package.json 
```

I was reading through `@phosphor-icons/react` package.json files and saw that that `exports` field only allows root import. Adding a new subpath fixed the issue:
```json
{
  "exports": {
    ".": {
      "import": "./dist/index.es.js"
    },
    "./*": {
      "import": "./dist/icons/*.es.js"
    }
  },
}
```

I suggest adding this path to the `exports` field to allow users opt-in to the modularized imports if they want. I believe it's generally a good practice to allow this.
