Add onix-based flake, associated make task, and .envrc template

[emeinhardt]

Prelude to a minor bugfix PR in imandra-document + low-stakes example of a simple onix-based flake.

[actionshrimp]

Cool - this looks handy for a quick testing setup as you say and nice one figuring out the onix setup here.

We briefly touched on this when we were disucssing on Friday, but I now realise that although pinning the ocaml version to 5.2.0 is fine for compiling imandra-document with dune, when imandra-document is loaded into imandra for reasoning purposes it will need to be compatible with Imandra's OCaml version, which is currently 4.12.1.

So it is probably a helpful test to keep this dev setup's ocaml version to 4.12.1 too, to ensure that any changes to imandra-document still work on older OCaml versions and therefore remain compatible with imandra.

Otherwise looks good!

[actionshrimp]

Actually your point about LSP has made me reconsider this, pinning to 4.12 will give us a really old LSP version as they tend to enforce that newer LSPs build on newer ocaml versions which can be slightly annoying! So for a local setup, I think keeping to a newer ocaml version makes sense.

Ideally imandra-document would have a regular ocaml CI step (something like this) that actually installs it on the correct 4.12 version via opam, as that's what the imandra-installer does.

edit: nevermind, there already is one! we just need to add 4.12 to the list I believe.

[emeinhardt]

I've made the changes we discussed, [EDIT] and now nix run .#onix-lock-dev runs as expected. To do that I just needed to make sure the sed command explicitly sets a lockfile; without that, the default behavior of the onix CLI is to modify onix-lock.json.

[emeinhardt]

Per the onix readme, the API might change, so I've pinned the version here.