feat: Add `gl-m*-n2` utility classes

What does this MR do?

Adds gl-mx-n2 and gl-my-n2 utility classes. Will be used in gitlab!46907 (diffs)

Does this MR meet the acceptance criteria?

Conformity

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• 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
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