Chore: add CI workflow for lint, test, build

[asgeija]

Runs lint, format check, tests and build on every pull request to main and on pushes to main, so broken code is caught before review.

Uses npm ci to match the lockfile, caches dependencies and cancels outdated runs to save runner time.

Refs #45

[thortve]

@asgeija is there now a branch ruleset with status check requirement?

[thortve]

@asgeija maybe look into changing the prettier format from a simple check to a more sophisticated solution where the runner automatically commits the prettier formatting. Removes one less tedious task from out backs, and also results in more status checks passed. Making the CI more streamlined.

[asgeija]

@asgeija is there now a branch ruleset with status check requirement?

Not yet. If we want to, the status check can only be selected after it has run once,
so we can add the ruleset after this PR is merged and the first run
has passed. It will require the checks job on main.

[asgeija]

@asgeija maybe look into changing the prettier format from a simple check to a more sophisticated solution where the runner automatically commits the prettier formatting. Removes one less tedious task from out backs, and also results in more status checks passed. Making the CI more streamlined.

Thanks, but I'd rather not have the runner commit. A new commit
dismisses approvals, and commits pushed with the default token don't
re-trigger the workflow, so the required check could go missing. We can
later add a .vscode/settings.json with format on save instead, and keep
format:check in CI as the safety net.

[vitaliyb]

Good, ready to merge.

[asgeija]

@asgeija is there now a branch ruleset with status check requirement?

There is now a ruleset on main that requires the checks job