feat: Guild Integrations

[kyranet]

Please describe the changes this PR makes and why it should be merged:

This PR adds the ability inside discord.js to manage guild integrations, from fetching them to editing and deleting them. It also adds a new structure: Integration, for a better OOP management (integrations are fairly complex: structure).

Full documentation for integrations: https://discordapp.com/developers/docs/resources/guild#get-guild-integrations

Edit:

Turns out there is a GUILD_INTEGRATIONS_UPDATE event: https://discordapp.com/developers/docs/topics/gateway#guild-integrations-update
This has been implemented in this PR too.

Question: In the first commit, I have covered everything the API support minus creating integrations since we do not have a IntegrationStore (and since they're not provided during ready nor when fetching the guild, only when we fetch the integrations), should I add an createIntegration method in the Guild class to cover this? Resolved by creating Guild#createIntegration.

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.