feat(ci): gh actions release workflows for crates.io

currently crates.io releases are ad-hoc. let's add a gh action workflow that pushes `catgrad` and `catgrad-llm` to crates.io

can copy e.g https://github.com/hellas-ai/ws-mux-rs/blob/master/.github/workflows/release.yml

i run it on tag, so when you want to cut a new release- make a PR that bumps version numbers in `Cargo.toml`- merge it then tag it, crate gets pushed.

not urgent but needs doing at some point