Re: [whatwg/fetch] Add size limiter to `Body#formData` method (Issue #1592)

> Right now there is no way to call Body#formData on arbitrary user input without potentially causing an OOM


How about sinking a `clone()` of the request prior to calling `formData()`? 

```typescript
Deno.serve(async function(request) {
  try {
    const sink = new WritableStream(LimitedUnderlyingSink(10_000_000));
    await request.clone().body?.pipeTo(sink, { preventCancel: true });
    const _form = await request.formData(); 
    return new Response("OK\n");
  } catch (error) {
    return new Response(String(error) + '\n');
  }
});

function LimitedUnderlyingSink(maxByteLength: number): UnderlyingSink<ArrayBufferLike> {
  return {
    write(chunk) {
      maxByteLength -= chunk.byteLength;
      if (maxByteLength < 0) {
        throw new DOMException('Size limit exceeded', 'QuotaExceededError');
      }
    }
  }
}
```

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