Allow port forwarding visibility to be specified

Currently developers can specify which ports to forward, but they can't specify visibility of those ports in e.g. Codespaces, so users are manually setting them every time, which is a bit of a pain.

I would propose adding a `visibility: public | org | private` option to `portsAttributes`:

```json
{
...
"forwardPorts": [3000],
"portsAttributes": {
"3000": {
"visibility": "public"
}
}
...
}
```

C.f. https://github.com/github/feedback/discussions/10394