support async configResolved hook in plugins

[dominikg]

Clear and concise description of the problem

Plugins may want to do asynchronous setup based on information available from the configResolved hook.

usecase for me: reading svelte config with dynamic import sveltejs/vite-plugin-svelte#29

I'm willing to provide a PR with the needed changes.

Suggested solution

Make configResolved async and await the on the calls.

Alternative

Plugins could try to work around this by adding special handling to the first call of transform but that feels wrong.

Additional context

config hook has been made async recently: #2798 but it has less information available.