feat: configure whether or not to lazy load

I'm using an `ion-icon` in a dropdown interface, where a user has to click an element to show the dropdown's content. Because of how ionicons is implemented, the icons inside the dropdown are not loaded instantly because the request is performed when it is visible to the user.

This, of course, is not ideal, as it shows a flicker while the icons are not downloaded. Just like what happens when you set a `background-image` to an element in its `:hover` state.

Is there any way to force the load of invisible icons?

The issue names ion-icon and its visibility-triggered lazy loading, but provides no files or tests to start from. Trace the implementation that requests icons when they become visible, then define how configuration should allow invisible icons to load immediately and verify that dropdown icons no longer flicker.

索引モデルが issue の本文から書いたものです。