[Svelte] Improve typescript types + minor fixes

[jamesst20]

@reinink

I have rebased all my commits in my repo (https://github.com/jamesst20/inertia) to include the latest changes and afterwards I have taken all my packages/svelte files there to overwrite the ones in the current repo. I have stripped down all unnecessary changes and all new features as well.

This PR only improve the types and fix some possible regression that I will comment in a few seconds in the diff

Edit: @reinink it's ready. i'm done adding comments :)