Switch to vite-string-plugin (#25762)

Switch to
[`vite-string-plugin`](https://github.com/silverwind/vite-string-plugin)
to load SVGs as string during tests. The plugin will also be useful once
we switch to vite.

vitest.config.js

@@ -1,20 +1,6 @@
 import {defineConfig} from 'vitest/dist/config.js';
-import {readFile} from 'node:fs/promises';
-import {dataToEsm} from '@rollup/pluginutils';
-import {extname} from 'node:path';
 import vue from '@vitejs/plugin-vue';
-
-function stringPlugin() {
-  return {
-    name: 'string-plugin',
-    enforce: 'pre',
-    async load(id) {
-      const path = id.split('?')[0];
-      if (extname(path) !== '.svg') return null;
-      return dataToEsm(await readFile(path, 'utf8'));
-    }
-  };
-}
+import {stringPlugin} from 'vite-string-plugin';
 
 export default defineConfig({
   test: {