Add `compute` app function for running compute shaders #749

Job	Run time
nix-fmt-check	23s
nix-flake-check (ubuntu-latest)	27s
cargo (cargo fmt --verbose --all -- --check)	30s
nix-flake-check (macos-latest)	54s
cargo (cargo check --locked --workspace)	37s
cargo (cargo check --locked --examples --workspace)	37s
cargo (cargo test --locked --lib --bins --workspace --features "egui")	31s
cargo (cargo doc --locked --workspace --features "egui")	35s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	32s
cargo (mdbook build guide/)	35s
cargo (cargo test --locked -p book_tests)	28s
verifications	0s
guide-push-to-deploy	0s
cargo-publish	0s
	6m 9s

Provide feedback