chore: support Node 18 (matches Node-RED 4.x minimum)

[dxdc]

What

• engines.node lowered from >=20 → >=18.5, matching the Node-RED 4.x minimum and the contrib-node packaging spec recommendation:

Within the engines section of the package.json file you SHOULD declare the minimum version of Node that your package works on. This SHOULD satisfy the current minimum supported version of the latest Node-RED release.

• ci.yml test matrix gains 18 so the claim is verified, not asserted.

Caveats

• Node 18 is upstream EOL (2025-04-30). Users on 18 won't get Node security patches anymore. Node-RED still supports it though, and the spec is about runtime compatibility — so we follow the spec.
• Local sanity check passed on Node 22 (112/112). Node 18 verification is on CI for this PR.

What was checked locally

• jsonata@^2.0.5 declares engines.node >= 8 — fine on 18
• node-red-node-test-helper@^0.3.6 declares engines.node >= 14 — fine on 18
• No usage of features introduced after 18: no structuredClone callers, no Array.findLast, no top-level await, no native fetch in the runtime, node:fs/node:os/node:path prefixes are 16+

Test plan

• CI passes the test job on Node 18
• CI passes on Node 20, 22, 24 (regression check)
• Coverage job (Node 24) still publishes the artifact