We should use the json file extension for the stylelint config file.
Most configuration files used have a file extension that gives a hint on the type and structure of the content. This is also true for almost all linter config files in use. For the .stylelintrc this was not done, because until some more recent version of stylelint the file would not be recognized with .json extension. Now this seems fixed and I suggest changing the filename to use a file extension.
Furthermore, as stated on  the stylelint config can be in JSON, YAML or JS. As far as I know we use for Mediawiki projects only the JSON variant. With the file extension it is clear what format is used right away.