Add a Nix flake along with a package derivation (default.nix) and dev shell (shell.nix). Update hotglsl. #627

Job	Run time
nix-fmt-check	19s
cargo (cargo fmt --verbose --all -- --check)	59s
cargo (cargo check --locked --workspace)	3m 15s
nix-flake-check (ubuntu-latest)	20s
cargo (cargo check --locked --examples --workspace)	3m 12s
cargo (cargo test --locked --lib --bins --all-features --workspace)	6m 9s
nix-flake-check (macos-latest)	1m 21s
cargo (cargo doc --locked --all-features --workspace)	6m 26s
cargo (cargo test --locked -p nannou_core --no-default-features --features "libm serde")	1m 10s
cargo (cargo build --locked -p nannou --features "wasm-experimental" --target wasm32-unknown-unkn...	1m 49s
cargo (mdbook build guide/)	55s
cargo (cargo test --locked -p book_tests)	3m 15s
verifications	1s
cargo-publish	0s
guide-push-to-deploy	0s
	29m 11s

Provide feedback