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)

---------

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

public/img/svg/octicon-checklist.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-checklist" width="16" height="16" aria-hidden="true"><path d="M2.5 1.75v11.5c0 .138.112.25.25.25h3.17a.75.75 0 0 1 0 1.5H2.75A1.75 1.75 0 0 1 1 13.25V1.75C1 .784 1.784 0 2.75 0h8.5C12.216 0 13 .784 13 1.75v7.736a.75.75 0 0 1-1.5 0V1.75a.25.25 0 0 0-.25-.25h-8.5a.25.25 0 0 0-.25.25Zm13.274 9.537v-.001l-4.557 4.45a.75.75 0 0 1-1.055-.008l-1.943-1.95a.75.75 0 0 1 1.062-1.058l1.419 1.425 4.026-3.932a.75.75 0 1 1 1.048 1.074ZM4.75 4h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM4 7.75A.75.75 0 0 1 4.75 7h2a.75.75 0 0 1 0 1.5h-2A.75.75 0 0 1 4 7.75Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-checklist" width="16" height="16" aria-hidden="true"><path d="M2.5 1.75v11.5c0 .138.112.25.25.25h3.17a.75.75 0 0 1 0 1.5H2.75A1.75 1.75 0 0 1 1 13.25V1.75C1 .784 1.784 0 2.75 0h8.5C12.216 0 13 .784 13 1.75v7.736a.75.75 0 0 1-1.5 0V1.75a.25.25 0 0 0-.25-.25h-8.5a.25.25 0 0 0-.25.25Zm13.274 9.537v-.001l-4.557 4.45a.75.75 0 0 1-1.055-.008l-1.943-1.95a.75.75 0 0 1 1.062-1.058l1.419 1.425 4.026-3.932a.75.75 0 1 1 1.048 1.074ZM4.75 4h4.5a.75.75 0 0 1 0 1.5h-4.5a.75.75 0 0 1 0-1.5ZM4 7.75A.75.75 0 0 1 4.75 7h2a.75.75 0 0 1 0 1.5h-2A.75.75 0 0 1 4 7.75Z"/></svg>