Add a Nix flake along with a package derivation (default.nix) and dev shell (shell.nix). Update hotglsl. #625

Job	Run time
nix-flake-check (ubuntu-latest)	21s
nix-fmt-check	19s
cargo-verifications (cargo fmt --verbose --all -- --check)	56s
nix-flake-check (macos-latest)	1m 21s
cargo-verifications (cargo check --locked --workspace)	3m 4s
cargo-verifications (cargo check --locked --examples --workspace)	3m 5s
cargo-verifications (cargo test --locked --lib --bins --all-features --workspace)	6m 15s
cargo-verifications (cargo doc --locked --all-features --workspace)	6m 4s
cargo-verifications (cargo test --locked -p nannou_core --no-default-features --features "libm se...	1m 5s
cargo-verifications (cargo build --locked -p nannou --features "wasm-experimental" --target wasm3...	1m 57s
cargo-verifications (mdbook build guide/)	50s
cargo-verifications (cargo test --locked -p book_tests)	3m 17s
verifications	1s
guide-push-to-deploy	0s
cargo-publish	0s
	28m 35s

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