Add Settings Sync Code (Auto Sync)

[UnseenFaith]

Description of the PR

Adds back some old functionality from old Klasa (syncing users upon fetch via KlasaUserStore) and some of the other functionality we wanted from being able to override actions.

This needs my other PR on core to merge before this will build for Action#run to be async.

This also needs some discussion about User#settings, it seems not as trivial as guilds to sync.

Changes Proposed in this Pull Request (List new items in CHANGELOG.MD)

• KlasaClient#users: UserStore => KlasaUserStore
• KlasaUserStore#fetch => syncs settings

Semver Classification

• This PR only includes documentation or non-code changes.
• This PR fixes a bug and does not change the (intended) framework interface.
• This PR adds methods or properties to the framework interface.
• This PR removes or renames methods or properties in the framework interface.