Add exports entry point to support node's module resolution.

Nobody has claimed this yet.

React admin provides cjs and esm bundles. However, it relies on `main` and `module` in the package.json file to decide which version to use. It will work with most bundlers but fails when using node's module resolution as it doesn't support the `module` field. I was hit with this problem when trying to test a component using vitest.

Example:
```js
function ComponentA() {
return
}

function ComponentB() {
const { watch } = useFormContext()
return ...
}
```

SimpleForm is loaded using cjs and uses FormProvider (cjs) from react-hook-form.
ComponentB uses useFormContext from react-hook-form esm bundle and context will be null.

**Describe the solution you'd like**
Add `exports` to package.json specifying the entry points for `import` and `require`.

**Additional context**
https://github.com/vitest-dev/vitest/discussions/4233
https://nodejs.org/api/packages.html#package-entry-points

Start with the package.json entry points and the existing CJS and ESM bundles. Review Node's package entry-points guidance and reproduce the Vitest module-resolution case described in the issue. Done means import and require resolve the intended bundles without loading separate react-hook-form contexts.

Written by the indexing model from the issue text.