So of course after posting I actually figured out a way to do this but it seems a little hacky and if there is a better way to do it I’d love to know. My solution is as follows:

from(
  prompt in Prompt,
  select: %Prompt{
    id: prompt.id,
    promoted: prompt.promoted,
    promoted_at: prompt.promoted_at,
    published_at: prompt.published_at,
    status: prompt.status,
    short_id: prompt.short_id,
    title: prompt.title,
    title_slug: prompt.title_slug,
    playing: prompt.playing,
    seeking: prompt.seeking,
    message_length: prompt.message_length,
    time_scale: prompt.time_scale,
    interaction: prompt.interaction,
    response_speed: prompt.response_speed,
    setting: prompt.setting,
    user_id: prompt.user_id,
    inserted_at: prompt.inserted_at,
    updated_at: prompt.updated_at,
    score: fragment("(select count(*) from saved_prompts where prompt_id = p0.id) + (select count(*) from liked_prompts where prompt_id = p0.id) as score")
  },
  order_by: fragment("score desc"),
)