[v14] Provided variables docs are wrong, or doesn't support esmodules

I believe this is due to the Haste system used at Meta that handles JS modules differently, but the problem is this:

Following [the docs](https://relay.dev/docs/api-reference/graphql-and-directives/#provided-variables) I created a `UserIDArgumentProvider.relayprovider.js` file and used it in a fragment like so:

```
@argumentDefinitions(
id: {
type: "ID!"
provider: "../user/UserIDArgumentProvider.relayprovider"
}
)
```

(note the leading "../" on the provider path. It seems the argument provider import is relative to the output directory, which for me is `/__generated__`, so I needed to get back to `` but this is a tangent and might be worth a separate issue).

I then defined `user/UserIDProvider.relayprovider.js` as the docs indicate:

```
export default {
get(): string {
return "12345";
},
};
```

This compiles just fine, but the problem is at runtime, where this line of code fails:

https://github.com/facebook/relay/blob/17636d63796d6daf8ae9059d5a37f5609067f198/packages/relay-runtime/store/RelayConcreteVariables.js#L102

For my configuration, it should be:

```patch
-operationVariables[varName] = providedVariables[varName].get();
+operationVariables[varName] = providedVariables[varName].default.get();
```

However, I assume that won't work at Meta, so I guess it should support both. I remember this coming up with some other Relay feature and can't remember the resolution to that problem, so I leave that to the maintainers.

The alternative solution is to update the OSS docs to indicate that this is the correct way to write the provider:

```
export default function get(): string {
return "12345";
};
```

This version worked just fine for me.

I'm happy to put up a PR allowing a `default` or fixing the docs, but I'm not sure which is the right solution. Please let me know which way is preferred.