Add xisage.godot-engine to gallery

[xiSage]

Extension Submission

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Cdhecklist

• My folder follows the <author>/<extension-name> convention (lowercase, alphanumeric + hyphens only)
• I have added an extension.json with all required fields
• The id field in my extension.json matches my folder path (author.extension-name)
• I have added an icon file (PNG or SVG, under 100 KB)
• The icon field in extension.json matches my icon filename
• My extension is available at the install source I specified (winget/MS Store/URL)
• I have read the Contributing Guide

Additional context

[Niels Laute (niels9001)]

LGTM!