fix(stark-ui): include only "src" and "assets" folders in "lint-css" command

[christophercr]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

As a result of the PR #516, the new lint-css command in stark-core and stark-ui also lints the CSS files in the "coverage" and "reports" folders which are created by the unit tests. This causes the lint-css command to fail due to many violations which should not be in the scope of the linting.

What is the new behavior?

The lint-css command in stark-core and stark-ui only includes the files from the "src" and "assets" folders.

Does this PR introduce a breaking change?

[ ] Yes
[X] No