Split non-Devvit-specific helpers into a separate package

[eritbh]

Converts this project to a monorepo and adds an additional toolbox-storage package which contains all the non-Devvit-specific logic. This will let us use these helpers for Toolbox itself as well, which means we'll eliminate any issues stemming from implementation differences between this library and Toolbox itself.

This PR also actually exports the subreddit config classes from the new package - they were previously accessible via ToolboxClient methods, but the classes themselves weren't actually exposed for direct use.

Breaking changes:

• The toolbox-devvit package no longer exports most of the things it used to. The rest of them need to be imported from toolbox-storage instead if necessary; this should mostly impact Typescript consumers, who will want to add toolbox-storage to their devDependencies if they need to manually import its types.

TODO:

• figure out if we want to re-export the base classes/helpers from @toolbox-team/storage in toolbox-devvit
  • we dont
• make sure i set everything up correctly and publishing won't completely fail
  • figure out a versioning/tagging strategy
    • packages get individual versions; tags use the format packages/foo/v0.0.0; the "latest release" label on the releases page will just mean nothing
• add contributor/maintainer docs about testing stuff with --workspaces