Upgrade to Bevy 0.18

[jaydevelopsstuff]

This PR upgrades bevy to the current latest version (0.18) and implements all the necessary migration changes. It also upgrades wgpu to 27—in line with what Bevy 0.18 now uses.

Changes

• Added rust-toolchain.toml which explicitly pins the required Rust toolchain version to nightly

bevy_gpu_compute

• Use despawn instead of despawn_recursive and remove DespawnRecursiveExt trait imports (0.15 → 0.16)
• Use IntoScheduleConfigs trait instead of IntoSystemConfigs (0.15 → 0.16)
• Use Projection::Orthographic(OrthographicProjection { /* ... */ }) instead of just OrthographicProjection { /* ... */ } in spawn_fallback_camera (0.15 → 0.16)
• Mark WgslCode::from_string and WgslCode::from_file as unsafe since RenderDevice::create_shader_module is now marked as an unsafe function (0.15 → 0.16)
• Wrapped the WgslCode::from_string call within BevyGpuComputeTask::from_shader in an unsafe block and added // SAFETY: wgsl_shader_module is compiled and validated. This is sort of an assumption I made so if the maintainer could verify that that safety comment is correct that'd be great.
• When calling RenderDevice::poll, use wgpu::PollType::wait_indefinitely() instead of wgpu::Maintain::Wait (wgpu 24 → 25)

bevy_gpu_compute/examples

• Use MessageWriter instead of the removed EventWriter (0.16 → 0.17)
• Use Projection::Orthographic(OrthographicProjection { /* ... */ }) instead of just OrthographicProjection { /* ... */ } (0.15 → 0.16)