Integration Events with IncludeSender := true should suggest `var` to subscribers

When we add a subscriber with Shift+Alt+E to an event that declares true for IncludeSender, we only get the other, explicitly written parameters as arguments to our resulting subscriber function. We can then use Ctrl+Space within the argument list parentheses to see what other arguments are available and add them if desired.

However, when added this way, the sender is declared without var. So, (a) we have to add that ourselves, and (b) it might mislead someone into thinking they don't have the option of modifying the sender, but we do. Might a better default be to insert var Sender there? Then those who don't need/want to modify can drop the var. Afaict, this is how other args work.

Start at the Shift+Alt+E subscriber-generation path for events with IncludeSender := true and inspect the generated subscriber signature. Compare sender handling with the existing explicit-parameter behavior; done means the generated sender parameter includes var while other available arguments remain discoverable.