types: fix cell.prop object types (#1729)
types: fix cell.prop object types (#1729)

Fixes

```ts
const cell = new dia.Cell<{foo: string, bar: string}>({foo: 'foo', bar: 'bar'})

cell.prop({foo: 'foo2'}) // this would fail because it was asking for the bar property
```
clientIO/joint,kumilingus/joint,clientIO/joint,kumilingus/joint,kumilingus/joint,clientIO/joint,clientIO/joint,kumilingus/joint