feat: add a default types in the $$Generic

[fedorovvvv]

Hi!✨
A common problem with $$Generic is that there was no way to set a default type

I tried to add more functionality and now it is possible to set the default type as the second argument to $$Generic

type T = $$Generic<number, 2>
interface $$Props {
    value?:T
    result?: T extends 2 ? 'AAAAAA' : 'OOOOO'
}
export let value:$$Props['value'] = 2

I think this will take types in svelte one step further and open up a lot of new possibilities
I can't live without default types))🥲

[fedorovvvv]

I fixed the condition, error output and ran all the tests. Everything's fine.

I hope you will accept this PR or we will improve it together💛

[fedorovvvv]

No one has used this syntax yet because of the error condition, so the chance of affecting projects is minimal if not 0

[dummdidumm]

We're currently leaning towards going with the generics attribute solution instead, which means $$Generics would be deprecated. Given that I'm hesitant to merge this - with the generics attribute, this is straightforward to express:

<script generics="T extends number = 2">
  interface $$Props {
    value?:T
    result?: T extends 2 ? 'AAAAAA' : 'OOOOO'
  }
  export let value:$$Props['value'] = 2
</script>

Or using runes:

<script generics="T extends number = 2">
  let { value = 2, result } = $props<{
    value?:T
    result?: T extends 2 ? 'AAAAAA' : 'OOOOO'
  }>();
</script>

[fedorovvvv]

Whoa!
I wasted an evening digging up $$$Generic😂

Is generics written about in the documentation?

Thanks💜

[dummdidumm]

No official documentation yet, but it's described in the RFC

[fedorovvvv]

Wow, these are absolute new solutions for types, thank you)

I'm surprised I haven't seen this before