feat(Typography): add text size scale variables

What does this MR do?

This MR is part of series of MRs to make GitLab use the new type scale for headings Typography > Type scale (gitlab-org/gitlab-services&15).

This MR adds the underlying sizes to be used for typography across GitLab.

In an effort to keep MRs small, these variables are not used as part of this MR.

The goal is for all typography to point back to this scale eventually. Either directly, or through different levels of abstraction and aliasing. For example alert-heading → h4 (styles) → $gl-text-font-size-400-dynamic, but exact naming and process TBD.

The styles were tested in Figma and in a temporary project.

Underlying scale	Heading values

Next steps

Here is the plan for future MRs, but comments and advice appreciated.

• Add text size variables to gitlab-ui THIS MR
• Add additional variables or mixins needed for heading letter spacing (-0.01em) and line height (1.25 & 1.125). feat(Typography): add line-height and letter-sp... (!3513 - merged)
• Set headings to use the new metrics.
• Update components to use new heading styles.
• Integration #2244

Screenshots or screen recordings

No visible changes

Integration merge requests

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

Does this MR meet the acceptance criteria?

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• Code review guidelines.
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• 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:

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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.