feat: support Node 24

[gavmck]

Why

Node 24 is the current LTS. Downstream consumers moving to it (e.g. the Admiral Design System bumping its .nvmrc/engines to Node 24) hit an npm warn EBADENGINE because this package's engines.node tops out at ^22:

npm warn EBADENGINE Unsupported engine {
npm warn EBADENGINE   package: '@etchteam/stylelint-config@1.7.2',
npm warn EBADENGINE   required: { node: '^18 || ^20 || ^22' },
npm warn EBADENGINE   current: { node: 'v24.16.0', npm: '11.16.0' }
npm warn EBADENGINE }

What

• Add ^24 to engines.node.
• Add 24 to the CI lint matrix so support is actually exercised.

Verification

Ran npm ci + npm test (stylelint over ./test) locally on Node v24.16.0 — passes clean. The new matrix entry verifies it in CI.

feat: so semantic-release cuts a new minor that consumers can pin to.

[JoshTheWanderer]

Could we also drop 18 and 20, which are EOL now?

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud