Enable number-leading-zero in stylelint (LeadingZero in scss-lint)

Enables number-leading-zero in stylelint (and transitions from exclude_zero to include_zero in LeadingZero of scss-lint ).

We need to change LeadingZero of scss-lint from exclude_zero to include_zero as the default of @gitlab/stylelint-config (stylelint) is now "number-leading-zero": "always".

Configuration of @gitlab/stylelint-config: https://gitlab.com/gitlab-org/frontend/gitlab-stylelint-config/-/blob/052570fdbf85814a6877e54e04a3367ab234cc38/index.js#L43

Upstream docs: https://stylelint.io/user-guide/rules/number-leading-zero