fix(Typings): GuildPreview#features and Integration#type

[cherryblossom000]

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

• GuildPreview#features is GuildFeatures[] instead of GuildFeatures
• Integration#type is string instead of number
  This is what the types should be and what they are in the documentation.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

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.