feat: n/no-unsupported-features/node-builtins ignore fetch

[Global fetch](https://nodejs.org/api/globals.html#fetch) has been
supported without the `--experimental-fetch` CLI flag since Node.js
18.0.0.  Although it was considered experimental until 21.0.0, there
are no functional changes that I am aware of.  I see no risk of using it
on projects targeting Node.js >=18.

FIXME: fetch should not be ignored for Node.js <18, but there does not
appear to be a way to configure this.

Signed-off-by: Kevin Locke <[email protected]>

rules/n.js

@@ -15,6 +15,14 @@ module.exports = {
 
     // allow process.exit()
     // this rule is redundant with the built-in no-process-exit rule
-    "n/no-process-exit": "off"
+    "n/no-process-exit": "off",
+
+    // disallow unsupported Node.js built-ins
+    // Ignore fetch experimental warning on Node.js >=18 <21
+    "n/no-unsupported-features/node-builtins": ["error", {
+      "ignores": [
+        "fetch"
+      ]
+    }]
   }
 };