This PR adds a built-in adapter for the basedpyright language server.

For now, it's behind the `basedpyright` feature flag, and needs to be
requested explicitly like this for staff:

```
  "languages": {
    "Python": {
      "language_servers": ["basedpyright", "!pylsp", "!pyright"]
    }
  }
```

(After uninstalling the basedpyright extension.)

Release Notes:

- N/A