ci: test the Node binding on the 22/24 LTS (18/20 are EOL) (#294)

* ci: test the Node binding on the 22/24 LTS (18/20 are EOL)

Node 18 is EOL and 20 reaches EOL; move the binding CI matrix from [18, 20]
to [22, 24] (both LTS), bump the fixed Node setups in ci.yml/release.yml to 22,
and raise package.json engines to >= 20.

The N-API 8 binary is ABI-stable across Node versions, so no per-version build
is needed. The `npm test` script (`node --test __tests__/`) breaks on Node 22
— it resolves `__tests__` as a module — so switch to `node --test` (auto-
discovery). Verified locally on Node 22: build + 584 tests green.

* build(node): sync package-lock engines.node to >= 20

* ci(node): drop the __tests__/ path arg so node --test auto-discovers

On Node 22+, `node --test __tests__/` resolves the directory as a single
module and fails with one unrunnable subtest. `node --test` (no path)
auto-discovers every *.test.js under the package, matching the package.json
test script. Verified locally: 584 tests pass.

bindings/node/package-lock.json

@@ -12,7 +12,7 @@
         "@napi-rs/cli": "^2.18.0"
       },
       "engines": {
-        "node": ">= 18"
+        "node": ">= 20"
       },
       "optionalDependencies": {
         "wickra-darwin-arm64": "0.9.0",

bindings/node/package.json

@@ -44,7 +44,7 @@
     }
   },
   "engines": {
-    "node": ">= 18"
+    "node": ">= 20"
   },
   "optionalDependencies": {
     "wickra-linux-x64-gnu": "0.9.0",
@@ -60,7 +60,7 @@
     "artifacts": "napi artifacts",
     "universal": "napi universal",
     "version": "napi version",
-    "test": "node --test __tests__/",
+    "test": "node --test",
     "bench": "node benchmarks/throughput.js"
   },
   "devDependencies": {