fix(RoleManager): fix ID return value, change return type to collection

[vaporoxx]

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

1. When using RoleManager#fetch with a single ID, it tries to get the role from this.cache, however that won't work when cache is set to false and it will return null even when the role was fetched correctly. This has ben fixed by getting the role from the collection instead.

2. Unlike any other manager, fetching without passing an ID returns the manager instead of the fetched collection, this has been changed for consistency.

Status

• Code changes have been tested against the Discord API
• I know how to update typings and have done so

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.