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-diff-ignored.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-diff-ignored" width="16" height="16" aria-hidden="true"><path d="M13.25 1c.966 0 1.75.784 1.75 1.75v10.5A1.75 1.75 0 0 1 13.25 15H2.75A1.75 1.75 0 0 1 1 13.25V2.75C1 1.784 1.784 1 2.75 1ZM2.75 2.5a.25.25 0 0 0-.25.25v10.5c0 .138.112.25.25.25h10.5a.25.25 0 0 0 .25-.25V2.75a.25.25 0 0 0-.25-.25Zm8.53 3.28-5.5 5.5a.749.749 0 0 1-1.275-.326.749.749 0 0 1 .215-.734l5.5-5.5a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-diff-ignored" width="16" height="16" aria-hidden="true"><path d="M13.25 1c.966 0 1.75.784 1.75 1.75v10.5A1.75 1.75 0 0 1 13.25 15H2.75A1.75 1.75 0 0 1 1 13.25V2.75C1 1.784 1.784 1 2.75 1ZM2.75 2.5a.25.25 0 0 0-.25.25v10.5c0 .138.112.25.25.25h10.5a.25.25 0 0 0 .25-.25V2.75a.25.25 0 0 0-.25-.25Zm8.53 3.28-5.5 5.5a.749.749 0 0 1-1.275-.326.749.749 0 0 1 .215-.734l5.5-5.5a.751.751 0 0 1 1.042.018.751.751 0 0 1 .018 1.042Z"/></svg>