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-command-palette.svg

@@ -1 +1 @@
-<svg viewBox="0 0 16 16" class="svg octicon-command-palette" width="16" height="16" aria-hidden="true"><path d="m6.354 8.04-4.773 4.773a.75.75 0 1 0 1.061 1.06L7.945 8.57a.75.75 0 0 0 0-1.06L2.642 2.206a.75.75 0 0 0-1.06 1.061L6.353 8.04ZM8.75 11.5a.75.75 0 0 0 0 1.5h5.5a.75.75 0 0 0 0-1.5h-5.5Z"/></svg>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 16 16" class="svg octicon-command-palette" width="16" height="16" aria-hidden="true"><path d="m6.354 8.04-4.773 4.773a.75.75 0 1 0 1.061 1.06L7.945 8.57a.75.75 0 0 0 0-1.06L2.642 2.206a.75.75 0 0 0-1.06 1.061L6.353 8.04ZM8.75 11.5a.75.75 0 0 0 0 1.5h5.5a.75.75 0 0 0 0-1.5h-5.5Z"/></svg>