Adapter api for dev mode support

[JeanJPNM]

Describe the problem

#2051 allows us to integrate svelte kit into a node server, but it only currently works in production mode. While this is an excelent change, we still need to be able to use it on dev mode. #2426 also requires dev mode support for ALL adapters that opt to support that functionality.

Describe the proposed solution

Making adapters able to optionally export a plugin with additional methods to use svelte kit specific functionality.

The plugin would have to be explicitly applied by the user. This allows the adapter to optionally receive custom parameters from the user and lets the user decide the order of application of all vite plugins being used.

Alternatives considered

none

Importance

i cannot use SvelteKit without it

Additional Information

• requires #2232
• requires exposing a way for adapters to start the dev mode
• likely another breaking api change