Alias with -st-extend shouldn't be possible

[idoros]

Currently when adding -st-extends to an alias it:

1. warns for unscoped class, because we used it without any root scoping
2. creates a local class symbol because it's a simple class with -st-extends

I think it should allow the local definition first and error with Import declaration conflicts with local declaration of 'symbol name'

example:

@st-import [icon] from "./button.st.css";
@st-import OtherIcon from "./other-icon.st.css";

.icon { /* is this a local definition or an import alias? */
    -st-extends: OtherIcon;
} 

• playground example