add github action

[oliver-sanders]

September training day activity: TIL reusable actions.

Add a simple "setup-cylc" action to facilitate writing CI for Cylc workflow projects.

Namely:

cylc lint
cylc validate
rose macro -V
rose metadata-check

The test action on this PR won't run until merged. Example of successful run here

Example of intended usage (using oliver-sanders/setup-cylc) on a dummy workflow, here.

TODO:

• Tag / branch V1 post-merge
• Setup caching (future work - cache mamba environment #2)

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included - initial commit!
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[hjoliver]

Nice

[hjoliver]

(Is the TODO list above for post-merge?)