types: improve useSWRSubscription types

[promer94]

This pr intend to improve useSWRSubscription types.

close #2523

Example

• inline subscribe function

import useSWRSubscription from 'swr/subscription'
import type { SWRSubNext } from 'swr/subscription'

const useExample = () => {
  const { data, error } = useSWRSubscription('key', 
    (key, { next }: SWRSubNext<string, Error>) => {
     //^ key will be inferred as `key`
     //....
   })
   return {
      data,
      //^ data will be inferred as `string | undefined`
      error
      //^ error will be inferred as `Error | undefined`
   }
}

• declare subscribe function

import useSWRSubscription from 'swr/subscription'
import type { SWRSubscription } from 'swr/subscription'

// The first generic is Key
// The second generic is Data
// The Third generic is Error
const sub: SWRSubscription<string, string, Error> = (key, { next }) => {                         
  //......
}
const { data, error } = useSWRSubscription('key', sub)

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 8416cae:

Sandbox	Source
SWR-Basic	Configuration
SWR-States	Configuration
SWR-Infinite	Configuration
SWR-SSR	Configuration