Add threads support

[Bilal2453]

This should be everything we need to get Thread channels going on. Still some things to work out, mainly design related.
Here is an overview of the changes:

Added 2 new classes:

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• GuildThreadChannel: Represents a Thread channel, inherits from TextChannel.

• ThreadMember: Wraps a Guild Member in a context of a specific Thread channel. This contains things like thread join time, etc.

Modified classes:

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• Cache: the hash function used to get unique object keys is now exposed through Cache._hash, this allows us to prioritize using a specific field instead of data.id.

This is needed because Discord sends id in the ThreadMember data... but instead of that ID being the member ID, it is actually the thread's ID. Discordia will always use the field id as a hash when inserting into a cache, so we had to have a way to tell Discordia to use data._user_id instead of data.id.

• SecondaryCache: added SecondaryCache:_delete(k), allowing us to delete an object by key, instead of being forced to use _remove for when we don't have the data, only the key.

This is needed to clear stale threads from cache when Discord sends THREAD_LIST_SYNC event, which only provides the ID of the thread to be removed.

• Cache & SecondaryCache: allow passing a custom parent in :_insert.

A thread channel parent is the text channel it is created under, yet Discord will send the thread channels on GUILD_READY as guild.threads, this means threads needs to be cached in Guild and the text channels needs to hold a reference to this.

Therefor the thread channels cache is located and initialized at Guild._thread_channels, and each GuildChannel holds a SecondaryCache to that. A direct consequence of this is all thread channels will share the same parent instance (:_insert defaults to a single parent object provided at initialization). With this change it becomes possible to provide a different parent with each :_insert call.

New API calls:

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• getThreadMember.
• getThreadMembers.
• addThreadMember.
• removeThreadMember.
• getCurrentThreadMember.
• joinThread.
• leaveThread.
• startThreadWithMessage.
• startThreadWithoutMessaeg.
• listArchivedPublicThreads.
• listArchivedPrivateThreads.
• listJoinedArchivedPrivateThreads.

Those are supposed to be auto-generated... but we don't have the API generator available in the repo it seems? I vaguely remember a Discord docs scraper.

New resolvers

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• Resolver.channelFlag: similar to how messageFlag is handled.

• Resolver.isoTimestamp: utilize the Date class into timestamp inputs useful for getArchivedPublicThreads(..., before), which is an ISO timestamp input; accepts Date, Date.fromTable, Date.fromSeconds.

Also been thinking into integrating everything that accepts Time objects for duration as a resolver, we already make use of Time objects for Member.timeoutFor, and now Time is also used for things like GuildThreadChannel:setAutoArchiveDuration(duration).

New events:

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• threadCreate: fired when a new thread channel is created.
• threadUpdate: fired when a thread channel is updated.
• threadDelete: fired when a thread channel is deleted and is in cache.
• threadDeleteUncached: fired when a thread channel is deleted and is not in cache.
• threadListSync: fired when the client gains/loses access to threads (e.x. gains permissions) and should sync cache.
• threadMemberUpdate: when a thread member is updated (e.x. left/joined).
• threadMembersUpdate: multiple thread members updates.

[Bilal2453]

I think I am mostly satisfied with this. I still need to do more testing before it becomes ready for merge, but should be good for a technical review.

One thing I am really not sure if I should keep are the permissions methods.

GuildThreadChannel:canManage()/ThreadMember:canUnarchive() etc. I honestly added them without much thought as I was looking into other libraries. But they don't really feel like they fit with Discordia's style, I feel like if we added them here they should also be added in TextChannel, with something like TextChannel:canSend().

Also to note, notice the naming scheme for GuildThreadChannel.isPrivate while the rest of getters are named like archived/pinned without the is, if I am not mistaken only methods should be prefixed with is and properties would simply have ed suffix. But I noticed the already existing GuildTextChannel.isNews, so I just followed what we already had. Those type of properties (ie isNews/isThread) also feel like they should be in Channel instead of the class directly? Like the new Channel.isThread.