[Tech] Type-check Backend & Frontend events

[CommandMC]

This adds typechecking for backendEvents and sendFrontendMessage. I've already caught a few issues with this (see #3648, some smaller issues are also corrected here)
Typechecking is done with the usual pattern of an interface holding all possible events/messages, and their corresponding function definitions (similarly to how IPC works, for example)

This is entirely a dev change, no actual code is altered

Requires #3648

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[arielj]

looks good to me 👍