ci: block PRs on fmt/clippy/test

[Mic92]

I noticed there was no CI, so I added this CI workflow with cargo fmt --check, cargo clippy (non-blocking until the existing lints are cleaned up) and cargo test run on every PR, push to main, and merge-queue entry. This will make the test ci job required. We can use this ci check for automated github dependency updates

Here some instructions on how setup required checks after merging:

export REPO=raine/workmux

# allow PRs to use auto-merge
gh api -X PATCH "repos/$REPO" -f allow_auto_merge=true

# require the `test` check on the default branch via a ruleset
# integration_id 15368 = GitHub Actions, actor_id 5 = repo admin (bypass)
gh api -X POST "repos/$REPO/rulesets" --input - <<'JSON'
{
  "name": "main",
  "target": "branch",
  "enforcement": "active",
  "conditions": { "ref_name": { "include": ["~DEFAULT_BRANCH"], "exclude": [] } },
  "bypass_actors": [
    { "actor_id": 5, "actor_type": "RepositoryRole", "bypass_mode": "always" }
  ],
  "rules": [
    { "type": "deletion" },
    { "type": "non_fast_forward" },
    { "type": "required_status_checks",
      "parameters": {
        "strict_required_status_checks_policy": false,
        "required_status_checks": [
          { "context": "test", "integration_id": 15368 }
        ]
      }
    }
  ]
}
JSON