improve support for environment api

[dominikg]

Describe the problem

currently vite-plugin-svelte still uses the ssr flag passed to hooks, this only works due to vites fallback support.

Describe the proposed solution

switch to this.environment information and also see what we can do with the compile cache and possibly handleHotUpdate (ssr module invalidation)

Alternatives considered

keep as is, but it might lead to less optimal usage with environment api

Importance

would make my life easier