feat: use promises in callbacks + cleanup

[Z3rio]

Description

This PR adds the usage of promises in our callbacks. Meaning we can now trigger callbacks in this manner:

local response = QBCore.Functions.TriggerCallback("CALLBACK NAME", 123, "test value")

Without loosing compatibility with the old format:

QBCore.Functions.TriggerCallback("CALLBACK NAME", function(response)

end, 123, "test value")

This also introduces the QBCore.Shared.IsFunction function, to check whether a value is a function OR a funcref!

Only thing I'm a bit unsure about here, would be the fact that functions become funcrefs, which cant be checked via type(), as they return table instead of function. The solution here is a bit hacky, but the only real "good" way would be the PR fivem and add some kind of function to check whether a value is a funcref or not.

Checklist

• I have personally loaded this code into an updated qbcore project and checked all of its functionality.
• My code fits the style guidelines.
• My PR fits the contribution guidelines.

[Z3rio]

This has been partially tested by me. Any feedback will be greatly appreciated.
Especially any idea on how to better recognize funcrefs

This has now also been tested in-game by me.