Remove support for older Node.js versions than 14.18.0 (#23)

`np` (`npm run release`) now requires the `engines.node` field in `package.json`,
so this change adds the field. The minimum version is determined by `ls-engines`.

```console
$ npx ls-engines@latest
...
┌────────────────────────┬───────────────────────────┐
│ package engines:       │ dependency graph engines: │
├────────────────────────┼───────────────────────────┤
│ "engines": {           │ "engines": {              │
│   "node": ">= 14.18.0" │   "node": ">= 14.18"      │
│ }                      │ }                         │
└────────────────────────┴───────────────────────────┘
...
Your “engines” field exactly matches your dependency graph’s requirements!
```

CHANGELOG.md

@@ -3,6 +3,7 @@
 ## Head
 
 - Removed: Prettier 2 support.
+- Removed: support for older Node.js versions than `14.18.0`.
 - Changed: package type to pure ESM.
 
 ## 2.0.0

package.json

@@ -47,6 +47,9 @@
   "peerDependencies": {
     "prettier": ">=3.0.0"
   },
+  "engines": {
+    "node": ">=14.18.0"
+  },
   "publishConfig": {
     "access": "public"
   }