The following skins and extensions have a .eslintignore file, but it also possible to exclude the vendor and node_modules folder over grunt config in Gruntfile.js
I would say it is better to use Gruntfile.js to see the ignore at one place, because jsonlint or other build tools are using it too.
Pro argument is that IDE can use the file (I have no idea, if there is any IDE support for eslint)
It is okay to use eslintignore or should we decide to use Gruntfile.js? That would remove a file from the root path and define a default handling.
Thanks for comments
This could also be done for .jshintignore or other tools over grunt.
==== Skins ====
[x] Timeless
==== Extensions ====
[x] ArticlePlaceholder
[ ] DataTypes
[x] PropertySuggester
[ ] Wikibase
[ ] WikibaseJavaScriptApi
[ ] WikibaseLexeme
[x] WikibaseMediaInfo
[ ] WikibaseQuality
[ ] WikibaseQualityConstraints
[ ] WikibaseQualityExternalValidation
[x] Wikidata.org
[ ] WikimediaBadges