Provide a way to subset the vendored icon fonts

Reading the [documentation](https://docs.expo.dev/guides/icons/), from my naïve interpretation it seems like this library is embedding the entire font file into the bundle:

> As with [any custom font](https://docs.expo.dev/develop/user-interface/fonts#use-a-local-font-file) in Expo, you can preload icon fonts before rendering your app. The font object is available as a static property on the font component. So, in the case above, it is Ionicons.font, which evaluates to {ionicons: require('path/to/ionicons.ttf')}.

Typical users don't want to include the entire icon set, but just a few icons out of it. We can do subsetting manually and load it as a custom icon font, but it would be better if `@expo/vector-icons` can handle this scenario, maybe by providing a config plugin or somehow?

Start with the Expo icons documentation linked in the issue and inspect how the library exposes its font assets to Expo. Define and document a supported way to subset an icon font, such as a config plugin or another API, with the result verified by confirming that only selected icons are included in the app bundle.

Written by the indexing model from the issue text.