Proposal: frint-data interop with Observable.from

## Currently

With `frint-data`, Models and Collections can be observed via `.get$()`:

```js
const todo$ = todo.get$();
```

## Proposal

As it is done for `frint-store` via #393, it would be useful to make them interoperable with `Observable.from` as per the spec on TC39: https://github.com/tc39/proposal-observable#observablefrom

```js
const todo$ = Observable.from(todo);
```

Review the existing frint-store interoperability work in #393, then inspect how frint-data Models and Collections expose .get$(). Compare the integration with the TC39 Observable.from specification; done means Observable.from(todo) works for both Models and Collections.

Written by the indexing model from the issue text.