Support WordPress in a subdirectory

I am trying to add an existing site in a folder like `~/my-site`.

`my-site` uses WordPress in a subdirectory. This is the basic structure of `my-site`:

```
.
├── index.php
├── salt.php
├── wp
├── wp-cli.yml
├── wp-config.php
├── wp-content
└── xmlrpc.php
```

Studio does not recognize `my-site` as an existing WordPress folder.

Context: I use WordPress in a subdirectory so that I can use [the WordPress composer package](https://github.com/johnpbloch/wordpress) to manage which version of Core is installed.

It looks like the immediate cause for the error message is the logic in this function: https://github.com/Automattic/studio/blob/trunk/src/lib/fs-utils.ts#L47-L53

However I don't know if updating that function would be enough to fix the issue, there definitely could be other parts of Studio which could not be compatible with the subdirectory approach for sites.