fix: inject headers defined in vite config

[Nemikolh]

Hey there! 👋 Thanks for the amazing project! 🤩

This PR fixes an issue that I've run into when trying to use @webcontainer/api with slidev. The problem is that slidev ignores the server.headers configuration when serving the initial page.

After this PR, the headers field will now be respected for vite.config.ts similar to this:

import { defineConfig } from 'vite';

export default defineConfig({
  server: {
    headers: {
      'Cross-Origin-Embedder-Policy': 'require-corp',
      'Cross-Origin-Opener-Policy': 'same-origin',
    },
  },
});

For applying the headers from that field, I've followed the same logic as vite does for static assets. See here.

Let me know if there's anything I should do! 🙌

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