Keep (add if not existing) xmlns attribute for generated SVG images (#23410)

Fix #23409 

Developers could browse & preview the local SVG images files directly.

It still has clear output.

![image](https://user-images.githubusercontent.com/2114189/224317107-f4b26c76-e36a-4e80-9eee-d8dc2e16421f.png)


![image](https://user-images.githubusercontent.com/2114189/224317527-2d4ca131-978c-4933-b071-4bae483f06e1.png)

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Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: delvh <leon@kske.dev>

public/img/svg/octicon-image.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-image" width="16" height="16" aria-hidden="true"><path d="M16 13.25A1.75 1.75 0 0 1 14.25 15H1.75A1.75 1.75 0 0 1 0 13.25V2.75C0 1.784.784 1 1.75 1h12.5c.966 0 1.75.784 1.75 1.75ZM1.75 2.5a.25.25 0 0 0-.25.25v10.5c0 .138.112.25.25.25h.94l.03-.03 6.077-6.078a1.75 1.75 0 0 1 2.412-.06L14.5 10.31V2.75a.25.25 0 0 0-.25-.25Zm12.5 11a.25.25 0 0 0 .25-.25v-.917l-4.298-3.889a.25.25 0 0 0-.344.009L4.81 13.5ZM7 6a2 2 0 1 1-3.999.001A2 2 0 0 1 7 6ZM5.5 6a.5.5 0 1 0-1 0 .5.5 0 0 0 1 0Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-image" width="16" height="16" aria-hidden="true"><path d="M16 13.25A1.75 1.75 0 0 1 14.25 15H1.75A1.75 1.75 0 0 1 0 13.25V2.75C0 1.784.784 1 1.75 1h12.5c.966 0 1.75.784 1.75 1.75ZM1.75 2.5a.25.25 0 0 0-.25.25v10.5c0 .138.112.25.25.25h.94l.03-.03 6.077-6.078a1.75 1.75 0 0 1 2.412-.06L14.5 10.31V2.75a.25.25 0 0 0-.25-.25Zm12.5 11a.25.25 0 0 0 .25-.25v-.917l-4.298-3.889a.25.25 0 0 0-.344.009L4.81 13.5ZM7 6a2 2 0 1 1-3.999.001A2 2 0 0 1 7 6ZM5.5 6a.5.5 0 1 0-1 0 .5.5 0 0 0 1 0Z"/></svg>