feat(icons): added `shield-user` icon

[sebinemeth]

closes #2600

What is the purpose of this pull request?

• [x] New Icon

Description

Added new shield-user icon.

Icon use case

Access to admin area, mark user as admin/elevated access rights

Alternative icon designs

Icon Design Checklist

Concept

• [x] I have provided valid use cases for each icon.
• [x] I have not added any a brand or logo icon.
• [x] I have not used any hate symbols.
• [x] I have not included any religious or political imagery.

Author, credits & license

• [ ] The icons are solely my own creation.
• [x] The icons were originally created in #2600 by @sebinemeth
• [x] I've based them on the following Lucide icons: shield, user
• [ ] I've based them on the following design:

Naming

• [x] I've read and followed the naming conventions
• [x] I've named icons by what they are rather than their use case.
• [x] I've provided meta JSON files in icons/[iconName].json.

Design

• [x] I've read and followed the icon design guidelines
• [x] I've made sure that the icons look sharp on low DPI displays.
• [x] I've made sure that the icons look consistent with the icon set in size, optical volume and density.
• [x] I've made sure that the icons are visually centered.
• [x] I've correctly optimized all icons to three points of precision.

Before Submitting

• [x] I've read the Contribution Guidelines.
• [x] I've checked if there was an existing PR that solves the same issue.