Support easier individual imports of modules

## Currently

Everywhere, including documentation and examples, we ask developers to import modules from the package directly, like this:

```js
import { Region } from 'frint-react';

// this import also brings `render` and `streamProps` for example
```

Some applications may need only one specific module from a given package, but they will still end up loading all the modules from that package in the runtime.

We can possibly get around this by tree-shaking during the bundling process, but the application on the server will not benefit from it in any way.

## Area for improvement

Without breaking the API, allow developers to import only the modules they want individually, like this:

```js
import Region from 'frint-react/Region';
```

To do this, we need to add files in the root directory of the packages according to the exported module names listed in their main `index.js` file.

Start by inspecting each package's main index.js file and the exported module names it lists, then compare the package root structure. Done means individual imports such as frint-react/Region work without breaking the existing package-level API or requiring unrelated modules at runtime.

Written by the indexing model from the issue text.