Core(TypeScript): Augment Settings#get to accept an interface overload

[kyranet]

Technically, it's completely possible to overload it so it takes a special interface, as used by Node.js's util.promisify:

CustomPromisify definition:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/3ef1069095013311df0fc6df1cb1ba92c220be19/types/node/util.d.ts#L57-L59

CustomPromisify overload usage:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/3ef1069095013311df0fc6df1cb1ba92c220be19/types/node/util.d.ts#L86

Usage examples:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/3ef1069095013311df0fc6df1cb1ba92c220be19/types/node/fs.d.ts#L864-L892

This would allow us to do message.guild.settings.get(GuildSettings.Prefix); and get string as return type, likewise message.guild.settings.get(GuildSettings.Roles.Public); would get readonly string[] as return type.

So essentially we can forget about the headache of having to do message.guild.settings.get(GuildSettings.Roles.Public) as GuildSettings.Roles.Public; which I often declare as an argument to improve code readability.