JSON encoding fails on `attr` argument in `alert` modifier

When using the `alert` modifier with an `isPresented`:
```ex
alert(attr("title"), isPresented: attr("presented"), actions: :actions, message: :message)
```

Compilation fails with the following error:

```
** (ErlangError) Erlang error: {:unsupported_type, {:isPresented, {:__attr__, [file: "core_components.swiftui.ex", line: 1, module: "", source: "alert(attr(\"title\"), isPresented: attr(\"presented\"), actions: :actions, message: :message)"], "presented"}}}
(stdlib 6.1.2) json.erl:199: :json.do_encode/2
(stdlib 6.1.2) json.erl:244: :json.list_loop/2
(stdlib 6.1.2) json.erl:241: :json.do_encode_list/2
(stdlib 6.1.2) json.erl:244: :json.list_loop/2
(stdlib 6.1.2) json.erl:244: :json.list_loop/2
(stdlib 6.1.2) json.erl:241: :json.do_encode_list/2
(stdlib 6.1.2) json.erl:241: :json.do_encode_list/2
(stdlib 6.1.2) json.erl:257: :json."-do_encode_map/2-lc$^0/1-0-"/2
```

I don't think stylesheet compilation errors should ever prevent an app from compiling. Instead the styles should be ignored.

Reproduce the alert modifier example using attr("presented") and trace the JSON encoding failure shown in the compiler error. Inspect the stylesheet compilation path to determine where this unsupported value is handled; done means the app compiles successfully and the affected styles are ignored instead of raising an error.

Written by the indexing model from the issue text.