Consider renaming @server and @client for windows compatability

# Issue

The CLI currently accepts `@` arguments to pass different args to different builds. Unfortunately, `@` has a special meaning in powershell on windows which can result in some confusing errors. On windows, it can also mean [splatting](https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_splatting?view=powershell-7.5)

We could use another symbol like `?`.

## Workarounds

If you find this issue after running into the above error, surrounding `@server` or `@client` in quotes fixes the issue:
```sh
dx serve "@server" --server
```