feat(css): Add utilities for CI config graph.

What does this MR do?

Adds utils that will be used by the CI config preview graph. These are added based on our color system so it's to avoid having to add a specific color in our codebase that might get out of sync with our design system.

stroke-blue will be used for the graph lines when we want to highlight them on hover inset-green will be used for the graph job pills. I used to use another one, but it's not like the designs. shadow-blue will be used for the graph jobs when we want to highlight them on hover

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• 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.
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  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• 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