conditional compilation of async compat (#95)

Problems with the previous code:

- `async-compat` is necessary for headless mode (cli server rendering)
to load remote scenes from the API without blowing up. Without it, the
code complains about the lack of a Tokio runtime.

- Unfortunately, `async-compat` doesn't work with WASM and the entire
web build is broken by its presence.

This change introduces conditional compilation so `async-compat` is only
leveraged in headless mode.

studio/Cargo.toml

@@ -15,6 +15,7 @@ path = "src/main.rs"
 [[bin]]
 name = "studio-headless"
 path = "src/main-headless.rs"
+required-features = ["headless_cli"]
 
 [dependencies]
 bevy-inspector-egui = "0.22.0"
@@ -69,6 +70,7 @@ wasm = [
 	"bevy_rapier3d/wasm-bindgen",
 ]
 desktop = []
+headless_cli = []
 
 [dependencies.bevy]
 workspace = true