Add Icon > Superscript and subscript icon

Purpose

Add icon of subscript and superscript

Screenshots

Subscript

Default	Inverse	Indigo	Gray

Superscript

Default	Inverse	Indigo	Gray

Checklist

For a new icon and/or illustration:

• Link to related design issue: gitlab#332071 (closed)
• If this adds an icon ensure that it follows the new icon checklists.
  • See related issue Icon > Superscript and subscript icon (#318 - closed)
• If this adds an illustration ensure that it follows the new illustration checklists.

For third-party trademarks:

• If this adds or updates an icon or illustration that includes a third-party trademark, ensure that it follows the process for adding third-party trademarks to GitLab.

Closes #318 (closed)