ci: split stylelint job and integrate with Code Quality

What does this MR do?

1. Add Code Quality-formatted output to the Stylelint job when it is run in CI.
2. To achieve this, split Stylelint out from the main lint job. (Only one Code Quality report can be provided for each pipeline job.)

The process for integrating Stylelint is now documented (gitlab!174650 (merged)).

This is very similar to the change, recently merged, that configured ESLint to send its output to Code Quality. !4820 (merged)

Related issues

https://gitlab.com/gitlab-com/Product/-/issues/13766, gitlab#385110 (closed)

Screenshots or screen recordings

This MR is self-testing. 🙂

The Code Quality widget shows the new findings. These new findings are from Stylelint. For reference, here is the job report artifact: gl-code-quality-report__15_.json

The job also passes:

Integration merge requests

N/A

• GitLab: mr_url
• CustomersDot: mr_url
• Status Page: mr_url

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• Code review guidelines.
• GitLab UI's contributing guidelines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened (see integration merge requests above).
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.