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-zap.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-zap" width="16" height="16" aria-hidden="true"><path d="M9.504.43a1.516 1.516 0 0 1 2.437 1.713L10.415 5.5h2.123c1.57 0 2.346 1.909 1.22 3.004l-7.34 7.142a1.249 1.249 0 0 1-.871.354h-.302a1.25 1.25 0 0 1-1.157-1.723L5.633 10.5H3.462c-1.57 0-2.346-1.909-1.22-3.004L9.503.429Zm1.047 1.074L3.286 8.571A.25.25 0 0 0 3.462 9H6.75a.75.75 0 0 1 .694 1.034l-1.713 4.188 6.982-6.793A.25.25 0 0 0 12.538 7H9.25a.75.75 0 0 1-.683-1.06l2.008-4.418.003-.006a.036.036 0 0 0-.004-.009l-.006-.006-.008-.001c-.003 0-.006.002-.009.004Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-zap" width="16" height="16" aria-hidden="true"><path d="M9.504.43a1.516 1.516 0 0 1 2.437 1.713L10.415 5.5h2.123c1.57 0 2.346 1.909 1.22 3.004l-7.34 7.142a1.249 1.249 0 0 1-.871.354h-.302a1.25 1.25 0 0 1-1.157-1.723L5.633 10.5H3.462c-1.57 0-2.346-1.909-1.22-3.004L9.503.429Zm1.047 1.074L3.286 8.571A.25.25 0 0 0 3.462 9H6.75a.75.75 0 0 1 .694 1.034l-1.713 4.188 6.982-6.793A.25.25 0 0 0 12.538 7H9.25a.75.75 0 0 1-.683-1.06l2.008-4.418.003-.006a.036.036 0 0 0-.004-.009l-.006-.006-.008-.001c-.003 0-.006.002-.009.004Z"/></svg>