Default platform setting for multi-target applications

## Feature Request

In multi-target fullstack setups like the template with fullstack enabled the `Cargo.toml` must have features for each platform like this:
```toml
[features]
desktop = ["dioxus/desktop"]
web = ["dioxus/web"]
mobile = ["dioxus/mobile"]

server = ["dioxus/server"]
```

During a `web` build, two binaries are built: the server with the `server` feature enabled and the client with the `web` feature enabled.

`web` should not be a default feature flag because that would enable the `web` feature during the server build. That could cause issues with user code with web-only logic gated behind that flag.

## Implement Suggestion

The CLI could read from a `default_platform` config with either web, desktop, or mobile to determine what platform to serve if `--platform` argument is set

Restore the `default_platform` setting that was removed in https://github.com/DioxusLabs/dioxus/pull/2975