fix(plugin-legacy): dynamic import browserslist-to-esbuild

[bluwy]

Description

fix #15988 (comment)

browserslist-to-esbuild is ESM only, so we need to dynamically import it.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

/ecosystem-ci run

[patak-dev]

Thanks Bjorn!

It seems Qwik and Rakkas are failing because of a puppeteer issue. Yesterday they were working but it doesn't look like this is in our side:
https://github.com/vitejs/vite-ecosystem-ci/actions/runs/8005837497/job/21866210713#step:8:1253

[vite-ecosystem-ci]

📝 Ran ecosystem CI on 1c61e19: Open

suite	result	latest scheduled
analogjs	✅ success	✅ success
astro	✅ success	✅ success
histoire	✅ success	✅ success
ladle	✅ success	✅ success
laravel	✅ success	✅ success
marko	✅ success	✅ success
nuxt	✅ success	✅ success
nx	❌ failure	❌ failure
previewjs	✅ success	✅ success
qwik	❌ failure	✅ success
rakkas	❌ failure	✅ success
sveltekit	✅ success	✅ success
unocss	✅ success	✅ success
vike	✅ success	✅ success
vite-plugin-pwa	✅ success	✅ success
vite-plugin-react	✅ success	✅ success
vite-plugin-react-pages	✅ success	✅ success
vite-plugin-react-swc	✅ success	✅ success
vite-plugin-svelte	✅ success	✅ success
vite-plugin-vue	✅ success	✅ success
vite-setup-catalogue	✅ success	✅ success
vitepress	✅ success	✅ success
vitest	✅ success	✅ success