Support Partial Update for JSON Files

I noticed that currently the only way to perform a partial update on an object is by using the spread operator, as shown in the following example:

```ts
fs[id].info.$data = { ...fs[id].info.$data, somekey: fs[id].info.$data.somekey + 1 };
```

While this method works, it has the downside of incurring I/O overhead if cache is disabled, and it might not be as readable as some other alternatives.

I suggest adding the possibility of performing partial updates using the following syntax:

```ts
fs[id].info.$data.somekey += 1;
```

This would make the process of performing partial updates easier and more intuitive.

Start by tracing the TypeScript API that exposes fs[id].info.$data and how JSON writes are triggered; the issue provides no file or test names. Compare the requested nested mutation syntax with the current spread-based update and define tests for correctness and cache-disabled I/O before implementation.

Written by the indexing model from the issue text.