Add presence_count to GUILD_CREATE

[SinisterRectus]

The incoming change in #1528 adds HTTP guild properties approximate_member_count and approximate_presence_count for users who do not or cannot track these numbers with gateway events. For the sake of consistency, I recommend that presence_count be added to guilds received via the GUILD_CREATE event (member_count already exists). This would allow users to have an initial presence count without having to make an HTTP request for every guild on initialization and without having to request presences (assuming they have not omitted the GUILDS intent).