Support stylelint 16.0.0 and upwards

Coming from the drupal project, where we use your fine module in our GitLab CI pipelines, we discovered that it is not working with the recently release stylelint 16 and upwards.

This is most probably due to the changes in the changes in Node.js API stylelint.formatters object directly.

Here's an example of a failed run:

$ stylelint "**/*.css" --color --custom-formatter=node_modules/stylelint-formatter-gitlab/index.js
TypeError: formatters[STYLELINT_FORMATTER] is not a function
    at module.exports (/builds/issue/drupal-3407211/core/node_modules/stylelint-formatter-gitlab/index.js:102:41)
    at prepareReturnValue (file:///builds/issue/drupal-3407211/core/node_modules/stylelint/lib/prepareReturnValue.mjs:71:23)
    at standalone (file:///builds/issue/drupal-3407211/core/node_modules/stylelint/lib/standalone.mjs:271:17)

https://git.drupalcode.org/issue/drupal-3407211/-/jobs/461347

We hope you can release a version that supports stylelint 16 and upwards soon.