Serve modern format with bridge and vite

Currently the vite implementation in Bridge diverges somewhat from within Nuxt 3. This is a problem for a couple of different reasons.

1. it increases the maintenance burden and doubles potential bug-surface
2. it is misleading for people migrating to nuxt 3, because the experience and compatibility is subtly different

One example is using @vitejs/plugin-legacy. The context for using it is the need for native ES module support in browsers. Otherwise, adopting vite for Nuxt 2 would mean unsupporting browsers that historically Nuxt 2 has supported. See browser support table on MDN.

I propose we align the implementations of vite in both Bridge & Nuxt 3, including (where possible) either removing use of @vitejs/plugin-legacy or porting it to the Nuxt 3 vite implementation.

Related issues with @vitejs/plugin-legacy

• it will be tricky to add support for dynamic base urls - seen whilst tackling nuxt/bridge#89
• the polyfill is loaded async which means it may not have been executed before entry, leading to a fatal error: nuxt/bridge#129 - solution in separate PR

Start by comparing the Vite implementations in Bridge and Nuxt 3, then review the @vitejs/plugin-legacy context and the related issues nuxt/bridge#89 and nuxt/bridge#129. Done means the implementations are aligned where possible and the listed dynamic-base-URL and asynchronous-polyfill concerns are addressed.