Replace native/web features with platform-specific dependencies once Rust 1.51.0 releases

[daboross]

With the new feature resolver stabilizing in 1.51.0, we should be able to get rid of native and web features, and instead use something like:

[target.'cfg(target_arch = "wasm32)'.dependencies]
bevy = {version = "0.4.0", default-features=false}
bevy_webgl2 = {version="0.4.0", optional=true}

[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
bevy = {version="0.4.0", default-features=false, features = ["bevy_wgpu"]}

I'd like to submit a PR for this, but it won't work until the itarget feature part of RFC 2957 is stabilized, which as of rust-lang/cargo#8997 will be Rust 1.51.0.

[mrk-its]

done