cross-platform main function

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Android, iOS, and INSERT_FUTURE_PLATFORM_HERE each require different forms of "main function" boilerplate. The newly added #[bevy_main] proc_macro_attribute generates the required boilerplate for android and ios.

This is intended to make cross platform development more standardized / make the same bevy code work on all platforms.

#[bevy_main] is entirely optional. You don't need to include it if you are building for platforms that don't require special boilerplate.

#[bevy_main]
fn main() {
  App::build().run();
}

@simlay @enfipy @PrototypeNM1 @MichaelHills

(i tested this on desktop, android, and web. im testing on ios now)

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Just verified that iOS works too.