Backport code from @vitejs/plugin-legacy to vite_plugin_legacy

[Antti]

Description 📖

Backport most of the logic from @vitejs/plugin-legacy to vite_plugin_legacy

This is currently used in production for over a year to support a wide variety of legacy browsers.

Background 📜

@vitejs/plugin-legacy does a lot of magic to support legacy browsers by rewriting index.html,
we're mostly interested in:

• Safari 10 nomodule fix
• A workaround for browsers that support modules, but do not support dynamic import:
  • script has type module, so only browsers which support modules would load it
  • if the dynamic import works - nothing happens
  • if the dynamic import fails - loads polyfills legacy
  • when polyfills legacy is loaded - it will then execute the entrypoint

[alexookah]

@ElMassimo Can we review and merge this?