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>

build/generate-svg.js

@@ -25,14 +25,22 @@ async function processFile(file, {prefix, fullName} = {}) {
     if (prefix === 'octicon') name = name.replace(/-[0-9]+$/, ''); // chop of '-16' on octicons
   }
 
+  // Set the `xmlns` attribute so that the files are displayable in standalone documents
+  // The svg backend module will strip the attribute during startup for inline display
   const {data} = optimize(await readFile(file, 'utf8'), {
     plugins: [
       {name: 'preset-default'},
-      {name: 'removeXMLNS'},
       {name: 'removeDimensions'},
       {name: 'prefixIds', params: {prefix: () => name}},
       {name: 'addClassesToSVGElement', params: {classNames: ['svg', name]}},
-      {name: 'addAttributesToSVGElement', params: {attributes: [{'width': '16'}, {'height': '16'}, {'aria-hidden': 'true'}]}},
+      {
+        name: 'addAttributesToSVGElement', params: {
+          attributes: [
+            {'xmlns': 'http://www.w3.org/2000/svg'},
+            {'width': '16'}, {'height': '16'}, {'aria-hidden': 'true'},
+          ]
+        }
+      },
     ],
   });