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-heart-fill.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-heart-fill" width="16" height="16" aria-hidden="true"><path d="M7.655 14.916v-.001h-.002l-.006-.003-.018-.01a22.066 22.066 0 0 1-3.744-2.584C2.045 10.731 0 8.35 0 5.5 0 2.836 2.086 1 4.25 1 5.797 1 7.153 1.802 8 3.02 8.847 1.802 10.203 1 11.75 1 13.914 1 16 2.836 16 5.5c0 2.85-2.044 5.231-3.886 6.818a22.094 22.094 0 0 1-3.433 2.414 7.152 7.152 0 0 1-.31.17l-.018.01-.008.004a.75.75 0 0 1-.69 0Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-heart-fill" width="16" height="16" aria-hidden="true"><path d="M7.655 14.916v-.001h-.002l-.006-.003-.018-.01a22.066 22.066 0 0 1-3.744-2.584C2.045 10.731 0 8.35 0 5.5 0 2.836 2.086 1 4.25 1 5.797 1 7.153 1.802 8 3.02 8.847 1.802 10.203 1 11.75 1 13.914 1 16 2.836 16 5.5c0 2.85-2.044 5.231-3.886 6.818a22.094 22.094 0 0 1-3.433 2.414 7.152 7.152 0 0 1-.31.17l-.018.01-.008.004a.75.75 0 0 1-.69 0Z"/></svg>