ref: Update main to indicate it does not return

Sentry CLI's `main` function calls `commands::main`, which does not return, since it calls `std::process::exit`. Update the function signature to reflect this.

src/main.rs

@@ -4,6 +4,6 @@ mod config;
 mod constants;
 mod utils;
 
-pub fn main() {
+pub fn main() -> ! {
     commands::main()
 }