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-sidebar-collapse.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-sidebar-collapse" width="16" height="16" aria-hidden="true"><path d="M6.823 7.823a.25.25 0 0 1 0 .354l-2.396 2.396A.25.25 0 0 1 4 10.396V5.604a.25.25 0 0 1 .427-.177Z"/><path d="M1.75 0h12.5C15.216 0 16 .784 16 1.75v12.5A1.75 1.75 0 0 1 14.25 16H1.75A1.75 1.75 0 0 1 0 14.25V1.75C0 .784.784 0 1.75 0ZM1.5 1.75v12.5c0 .138.112.25.25.25H9.5v-13H1.75a.25.25 0 0 0-.25.25ZM11 14.5h3.25a.25.25 0 0 0 .25-.25V1.75a.25.25 0 0 0-.25-.25H11Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-sidebar-collapse" width="16" height="16" aria-hidden="true"><path d="M6.823 7.823a.25.25 0 0 1 0 .354l-2.396 2.396A.25.25 0 0 1 4 10.396V5.604a.25.25 0 0 1 .427-.177Z"/><path d="M1.75 0h12.5C15.216 0 16 .784 16 1.75v12.5A1.75 1.75 0 0 1 14.25 16H1.75A1.75 1.75 0 0 1 0 14.25V1.75C0 .784.784 0 1.75 0ZM1.5 1.75v12.5c0 .138.112.25.25.25H9.5v-13H1.75a.25.25 0 0 0-.25.25ZM11 14.5h3.25a.25.25 0 0 0 .25-.25V1.75a.25.25 0 0 0-.25-.25H11Z"/></svg>