tools: add `root: true` in main .eslintrc.yaml

This option prevents ESlint from unnecessary searching
in parent folders. It also protects ESlint rules
from accidental rewriting by a config in a parent folder.

PR-URL: https://github.com/nodejs/node/pull/12570
Fixes: https://github.com/nodejs/node/issues/12566
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Teddy Katz <teddy.katz@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

.eslintrc.yaml

@@ -1,3 +1,5 @@
+root: true
+
 env:
   node: true
   es6: true