Improvement: Remove `fs` from SSR bundle

[Valian]

Some server runtimes doesn't provide access to the filesystem, eg quickjs (see #125)

We use it only in one place - server.ts uses it to read manifest file and generate preload directives

There should be a way to configure Vite to embed manifest directly into SSR bundle. Some code I've found around that might be useful:

import fs from 'node:fs';
import path from 'node:path';
import { defineConfig } from 'vite';

export default defineConfig({
  build: {
    ssr: 'src/entry-server.js',
    // ... other config
  },
  define: {
    // Inject the manifest as a global constant
    __SSR_MANIFEST__: JSON.parse(
      fs.readFileSync(path.resolve(__dirname, './dist/client/.vite/ssr-manifest.json'), 'utf-8')
    )
  }
});

// entry-server.js
export async function render(url) {
  // __SSR_MANIFEST__ is now available as a global object
  const manifest = __SSR_MANIFEST__;
  
  // Use it to resolve assets as usual
  const appHtml = await renderApp(url);
  return { appHtml, manifest };
}

worth exploring, as it would make SSR rendering a bit less complex and faster.