refactor: v6

[Snazzah]

Making a bunch of changes in a major release to hopefully do some changes to slash-create that'll make it less bad. Some things like chaining methods together (on asynchronous calls) and handling command permissions internally makes it a bit of a mess, and I want to make slash-create more customizable. I'll chip away at this until it's good enough to release, since I tend to be busy a lot.

• Remove deprecated stuff
• Make important functions async (registering commands in a folder, syncing)
• Update request handler
• Remove command-related cruft (may include "reregistering")
• Refactor throttling to be easier to extend from (closes Ability to customise throttling a bit more #439)
• Add web exports for Cloudflare Workers (closes Consider improving support for not-node environments #539)
• Fix tests Node 18+: make nock work native fetch nock/nock#2397
• Use pnpm
• Changelog & documentation cleanup

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