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-accessibility.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-accessibility" width="16" height="16" aria-hidden="true"><path d="M9.923 5.302c.063.063.122.129.178.198H14A.75.75 0 0 1 14 7h-3.3l.578 5.163.362 2.997a.75.75 0 0 1-1.49.18L9.868 13H6.132l-.282 2.34a.75.75 0 0 1-1.49-.18l.362-2.997L5.3 7H2a.75.75 0 0 1 0-1.5h3.9a2.54 2.54 0 0 1 .176-.198 3 3 0 1 1 3.847 0ZM9.2 7.073h-.001a1.206 1.206 0 0 0-2.398 0L6.305 11.5h3.39ZM9.5 3a1.5 1.5 0 1 0-3.001.001A1.5 1.5 0 0 0 9.5 3Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-accessibility" width="16" height="16" aria-hidden="true"><path d="M9.923 5.302c.063.063.122.129.178.198H14A.75.75 0 0 1 14 7h-3.3l.578 5.163.362 2.997a.75.75 0 0 1-1.49.18L9.868 13H6.132l-.282 2.34a.75.75 0 0 1-1.49-.18l.362-2.997L5.3 7H2a.75.75 0 0 1 0-1.5h3.9a2.54 2.54 0 0 1 .176-.198 3 3 0 1 1 3.847 0ZM9.2 7.073h-.001a1.206 1.206 0 0 0-2.398 0L6.305 11.5h3.39ZM9.5 3a1.5 1.5 0 1 0-3.001.001A1.5 1.5 0 0 0 9.5 3Z"/></svg>