For example, if port 4000 is already in use, then this just uses another port.

```
$ quarto preview --port 4000
Check file:///Users/winston/quarto-cli/src/quarto.ts
Preparing to preview

Watching files for changes
Browse at http://localhost:7936/
```

It would be helpful to print a warning or throw an error.
2022-03-29T00:57:06Z
wch
closed
`quarto preview --port xxx` silently uses other port if port is already in use
https://github.com/quarto-dev/quarto-cli/issues/479