feat: Support slnf extension

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Purpose

Recently I added support for the new slnx extension (!1182 (merged)). Now I worked with solution filters and noticed that there is no icon for it as well.

This MR adds support for the .slnf format to the icon map. The .slnf file extension behaves like a normal .NET solution but only contains a subset (filter) of .NET projects.

Screenshots

Checklist

For a new icon and/or illustration:

• Link to related design issue:
• If this adds an icon ensure that it follows the new icon checklists.
• 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.