Create cfg aliases for bevy_app

[hukasu]

Objective

Implements partially feature requested in #1615

Solution

Create cfg aliases for some of the cfg used in bevy_app

Testing

Ran cargo run -p ci but needs more testing

Showcase

Before

#[cfg(all(any(unix, windows), feature = "std"))]
pub use terminal_ctrl_c_handler;
// ...
#[cfg(all(not(target_arch = "wasm32"), feature = "bevy_tasks"))]
bevy_tasks::tick_global_task_pools_on_main_thread();

After

#[cfg(std_windows_or_unix)]
pub use terminal_ctrl_c_handler::*;
// ...
#[cfg(can_run_tasks)]
bevy_tasks::tick_global_task_pools_on_main_thread();

[bushrat011899]

The inclusion of a build.rs is definitely going to be the controversial component of this PR. Between security and build time concerns, I see this single-handedly being a show-stopper.

Personally, I'm not against it, since we have many dependencies that use cfg_aliases already (which is effectively what you're doing here by hand). But I think this will be very hard to land.

[hukasu]

oh, cfg_aliases is already being used? i wanted to decrease the contentiousness by a little by not adding a new dependency, but if this is already present, maybe there won't be pushback on that point

[bushrat011899]

It's already brought in as a build-dependency by winit and wgpu, so I wouldn't say it's a controversial inclusion in of itself. It's purely the use of a build.rs that I see as being controversial.

[alice-i-cecile]

build.rs is a complete non-starter for me. That way lies madness!