Fix multi-extension entitlements

[russriser]

This PR fixes an issue where the entitlements for a pre-existing extension are overwritten.

Since the configIndex is set to 0 before writing the entitlements, we need to make sure we add the new extension at index 0, not to the end of the list.

[nathan-ahn]

@russriser Your code makes sense for fixing pre-existing extensions getting overwritten, but I can't figure out what the original code was intended to do. I might be reading your PR incorrectly, but it appears that this entire section now becomes useless

if (configIndex != null && config.extra) {
    const appClipConfig =
      config.extra.eas.build.experimental.ios.appExtensions[configIndex];
    appClipConfig.entitlements = {};
}

since appClipConfig (which I think should be widgetConfig) is never created with entitlements. Do you see any reason to not just delete the entirety of the above section?