maybe deno fmt should consider the presence of jsr.json a workspace

Currently if you have jsr.json (but no deno.json or package.json) and run deno fmt it shows this warning

```
deno fmt
Warning It looks like you're not in a workspace. Are you sure you want to format the entire '/home/mrcool/dev/deno/rustman' directory? [Y/n]
```

Maybe it shouldn't