fix(BaseManager): properly type valueOf

[almostSouji]

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

BaseManager#valueOf returns its cache collection, but the return is currently typed as :Object. This PR documents types the return value based on key and holdings.

Further considerations:

Whether or not valueOf should even return a Collection in the first place is another topic, which should be discussed for the next semver major. This is (according to @monbrey) the case due to Util#toJSON expecting to find Collections there to extract the appropriate data for use cases like broadcast evaluations.

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.