Add a Nix flake along with a package derivation (default.nix) and dev shell (shell.nix). Update hotglsl. #626

Job	Run time
nix-fmt-check	20s
cargo (cargo fmt --verbose --all -- --check)	1m 5s
nix-flake-check (ubuntu-latest)	24s
cargo (cargo check --locked --workspace)	3m 24s
nix-flake-check (macos-latest)	1m 23s
cargo (cargo check --locked --examples --workspace)	3m 13s
cargo (cargo test --locked --lib --bins --all-features --workspace)	6m 41s
cargo (cargo doc --locked --all-features --workspace)	6m 17s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	1m 17s
cargo (cargo build --locked -p nannou --features "wasm-experimental" --target wasm32-unknown-unkn...	2m 7s
cargo (mdbook build guide/)	1m 2s
cargo (cargo test --locked -p book_tests)	3m 25s
verifications	1s
guide-push-to-deploy	0s
cargo-publish	0s
	30m 39s

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