feature: catnip v3 and API v8 support

[queer]

As this makes some backwards-incompatible changes, this is a major version bump.

This is a WIP based off of discord/discord-api-docs#2097.

Note: The branch name is a little funny because I had started on v8 work, then realised it was full of breaking changes, so figured I'd do a version bump, so...

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An overview of changes

• API v8 is the minimum required version. All the related v8 changes have been added, see the linked PR above.
• Intents will actually warn properly if you're using them incorrectly now.
• API version and intents are sanity-checked.
• DM channels are no longer cached; they're fetched on-demand as needed.
• ALL cache-related methods, and anything that touches them, return Rx operators instead of entities/values directly.
• Member chunking is disabled by default.
• Missing guild features added.
• Support for stickers!!!!!!!!!!!!!!1 (Support stickers #490).
• More sanity checking of config.
• Missing message types added.
• Mention flags on edits (Support mention parse flags on message edit #435).
• Support for inline message replies (Add message type 19 REPLY #481, Support inline replies #484).
• Return Maybe instead of Single where it makes sense.
• Convenience method in RxHelpers to await a lot of Maybes and Singles.
• Add Message#reply().
• Rename RestChannel#sendMessage() to #createMessage() to match what's in the docs properly.
• Fix the examples to work with v3.
• Fix Member#mute()/deaf() (Member#mute()/deaf() don't work #489).
• Add proper support for new JSON errors.
• Decompose entities into interfaces that can reduce code duplication.
• Much better tests and the like via GitHub Actions.
• e2e testing to verify that (some) REST stuff actually works.
• Caches do the right thing without raising NPEs.
• Logging is a bit more configurable.
• Noop caches will no longer log spurious errors.
• Cache workers can now specify what entities they can provide history for. This fixes some DoubleEvent exceptions.
• CHANNEL_UPDATE is now a DoubleEvent the way it should have been.
• Cache flags are gone, as intents are an effective replacement for them.
• A lot more convenience methods!
• Support for inline replies.
• Interactions (currently just slash commands) are supported!!! (feature: interactions #500).
• Catnip#on() is now private, to push people towards using Rx operators properly.

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Q. Why does <this method> return an Rx operator, but <that method> returns an entity class?
A. This is due to how the Discord API works, and the points at which it does/not actually provide an entity.

Q. Returning Rx operators for every cache method is stupid and painful and bad you absolute moron.
A. That's not really a question, but unless you're doing some special caching stuff, you can wrap them in methods that block, or just use #blockingGet() everywhere, or... to get the old blocking behaviour. The reason for it being this way is because ex. a remote entity cache in Redis shouldn't block during the network round trips.

[queer]

APIv8 docs have been merged, so choo choo 🚂