BREAKING CHANGE: The `data` parameter of `VectorSource` has changed to
remove ambiguities between URLs and text data, and allow relative URLs.
Please refer to the documentation for more details.

For URL based sources:

```js
// Before
const source = new VectorSource({
    data: 'http://example.com/data.geojson',
    format: new GeoJSON(),
});

// Now
const source = new VectorSource({
    data: {
        url: 'http://example.com/data.geojson',
        format: new GeoJSON(),
    },
});
```

For content based sources (e.g a GeoJSON document):

```js
const geojson = JSON.parse(...);

// Before
const source = new VectorSource({
    data: geojson,
    format: new GeoJSON(),
});

// Now
const source = new VectorSource({
    data: {
        content: geojson,
        format: new GeoJSON(),
    },
});
```