Added Story/Meta typings for Svelte

[anthonator]

Issue: Svelte renderer doesn't have support for the Story and Meta types like React and Vue do. This results in Property 'args' does not exist on type type errors when using TypeScript in stories.

What I did

I added the missing typings.

How to test

• Is this testable with Jest or Chromatic screenshots? No
• Does this need a new example in the kitchen sink apps? No - the Svelte kitchen sink currently does not have TypeScript examples
• Does this need an update to the documentation? I don't think so. The only Storybook + Svelte + TypeScript documentation I could find was a blog. If I'm wrong I'm happy to update documentation if you point me in the right direction.

If your answer is yes to any of these, please make sure to include it in your PR.

[kylegach]

Hey, @anthonator. Thanks for this effort! And congratulations on your first contribution! 🥳

A couple things of note here:

1. Active development happens on the next branch, so your PR should target that.
2. The next branch is on 7.0 and the team has decided to not ship new features to 6.5, so only the types-7-0 file is necessary. Please look at the other renderer's types to see how the organization has changed between 6 & 7.

Thanks again for taking the time to improve Svelte types!

[ndelangen]

@anthonator could you please rebase on the next branch please? We cannot proceed with this PR otherwise. Thank you 🙏 !

[ndelangen]

@kasperpeulen perhaps this is relevant for your work on types?
If not, please disregard me ping-ing you.

[benmccann]

@ndelangen I'm afraid I don't know enough about Storybook to be able to review this PR. I'm happy to answer Svelte-specific questions though

[ndelangen]

@JReinhold perhaps you could take a peek at this one?

[JReinhold]

I think this looks like a pretty simple migration of React types to Svelte types so this is good! But I assume that @kasperpeulen's refactoring of the type system is going to have an effect on this, so I think his say is the most important.

[kasperpeulen]

@ndelangen The Svelte types are already in the 7 release, and I'm improving those this week.

So I guess the question is, do we want to backport those types to 6, otherwise, this PR is already done.

[kasperpeulen]

Starting on better CSF3 types for svelte in:
#19512