Github Action: Add Action to check code is formatted.

[KFoxder]

Overview

• Added simple GHA to check if code committed is formatted correctly. I found myself more than once opening a PR and then realizing there was so formatting required.

[leostera]

Thanks for the PR! What is the difference between that lint-fmt workflow you have and this change: cb8d348

It looks like both will fail when the code isn't properly formatted, but maybe I'm missing something else that lint-fmt does.

[KFoxder]

@leostera your change I think is functionally the same as what this PR is doing. I was just following what I saw in the official setup-ocaml action: https://github.com/ocaml/setup-ocaml?tab=readme-ov-file#extends.

Two quick things:

1. The commit you made though is breaking the builds now. I think the GHA I was using installs ocamlformat
2. Totally your decision but having the lint in its own GHA allows a developer to know if the build is working or not separate from knowing if they forgot to format their files before committing. Right now the build status is unknown for example since it is failing on the lint step which is before the build or tests in your commit. Either one is fine, really just your preference.