feat: add Piece and Store namespaces for easier importing types

[Lioness100]

Addon to #136. I'm not sure if I should make a new namespace for AliasStore, since it would literally be a duplicate of the Store namespace, or just accept it being inconsistent because AliasPiece has a namespace.

[favna]

I'm not sure if I should make a new namespace for AliasStore, since it would literally be a duplicate of the Store namespace, or just accept it being inconsistent because AliasPiece has a namespace.

I would say no. End-users won't really often be using AliasStore for their own code anyway because aliases are really only useful for message based commands. They're completely useless for context menu commands and pretty useless for slash commands.

(p.s. I have no idea what vladdy's plans are for handling aliases for slashies, or if he even gave that any thought yet)

[favna]

Changed the title / commit message because I realized there was no feat of this yet.