Update deprecated GitHub Actions (checkout, setup-node v2 -> v4)

[mattgodbolt]

Found by an org-wide actionlint sweep. Per review feedback the lint tooling itself is staying in the core repos (compiler-explorer, infra, compiler-workflows) — this PR keeps just the fixes, which stand on their own:

• actions/checkout v2 -> v4
• actions/setup-node v2 -> v4

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds GitHub Actions workflow linting via actionlint as part of an org-wide rollout, and updates existing workflows to use current major versions of core GitHub Actions.

Changes:

• Add a new workflow to run actionlint when workflow/config files change.
• Add an .github/actionlint.yaml configuration (runner labels + shellcheck gating).
• Bump actions/checkout and actions/setup-node to v4 in the Node CI workflow.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
.github/workflows/node.js.yml	Updates checkout/setup-node to v4 (but introduces/retains invalid steps: indentation that must be fixed).
.github/workflows/lint-actions.yml	Adds a workflow running actionlint on workflow/config changes.
.github/actionlint.yaml	Adds actionlint configuration (self-hosted runner labels + shellcheck ignore rules).

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[mattgodbolt]

Declining — this is valid YAML. Block sequence entries may sit at the same indentation column as their parent mapping key (a "zero-indented" block sequence); it parses identically to the extra-indented form. This workflow has parsed and run in exactly this form for years, and actionlint — which uses a real YAML parser — passes it cleanly.