Missing types in index.d.ts

It is great to have a type definitions file `index.d.ts`, but it looks like it has not been updated since a long time and is now quite incomplete.

In our codebase, we often use the imports listed below and they generate TS errors.

## Version

3.15.0

## Steps to reproduce

Add the following imports in a *.ts file

```ts
import { Changeset } from "ember-changeset";
import { EmberChangeset } from "ember-changeset";
```

## Expected Behavior

No TS error

## Actual Behavior

Module '"ember-changeset"' has no exported member 'Changeset'.
Module '"ember-changeset"' has no exported member 'EmberChangeset'.