No styles in toasts in additional windows (v11)

I’m trying to use react-toastify in a multi-window app (windows are open with `window.open` and render a subtree using React portals). But the styles from react-toastify are not injected in the other windows, making toasts only work in the main window.

React-toastify should detect the current window containing the toast container (`element.ownerDocument.defaultView`) and inject the styles there if they haven’t been injected already in that window. Alternatively, expose an `injectStyles` function (with a `window_` argument) to make it possible to inject the styles in a different window.

(already reported as #902 which was closed because it was supposed to be fixed in v11, but it isn’t)