Implement command to remove cached downloads

NOTE: this issue predates this project's rename to Volta.

This command is basically the inverse of notion fetch <node/npm/yarn/package> - it should remove the cached downloads for the specified tool from the inventory directory.

There was a discussion on discord, and several options for the syntax of this command were proposed:

• notion delete
• notion prune
• notion cleanup
• notion clear-cache

Whatever is chosen, there should be a corresponding flag for notion uninstall to also remove the cached files.