Add support for Steam Accounts in Shortcuts

Steam Rom Manager supports the freedom of scanning things in per account, meaning you can exclude some things from certain accounts, or include things in other accounts.

It would be great if Shortcuts could do the same thing. I have some games that have different launch commands depending on the User account, which allows us to have different game saves, etc.

Start by locating the existing Shortcuts implementation and the code that handles Steam account-specific scanning or launch commands. Define the expected account-selection behavior and its interaction with per-account inclusions, exclusions, and launch commands before determining what tests can demonstrate completion.

Written by the indexing model from the issue text.