Initial implementation of useSWRList

Implemented in accordance with the [RFC][1]. Initial implementation is
mostly a copy/paste of `useSWR`, then moved things into arrays so we
can handle the list aspect of this correctly. Plenty of work still to be done,
especially around the duplication between `useSWRList` & `useSWR`,
plus more tests need to be written, but this gives us a basis for discussion.

  [1]: #1988

_internal/index.ts

@@ -23,3 +23,5 @@ export { withMiddleware } from './utils/with-middleware'
 export { preload } from './utils/preload'
 
 export * from './types'
+
+export const WITH_DEDUPE = { dedupe: true }

_internal/types.ts

@@ -278,3 +278,15 @@ export interface StateDependencies {
   isValidating?: boolean
   isLoading?: boolean
 }
+
+export type DefinitelyTruthy<T> = false extends T
+  ? never
+  : 0 extends T
+  ? never
+  : '' extends T
+  ? never
+  : null extends T
+  ? never
+  : undefined extends T
+  ? never
+  : T

core/use-swr.ts

@@ -31,23 +31,11 @@ import {
   SWRConfiguration,
   SWRHook,
   RevalidateEvent,
-  StateDependencies
+  StateDependencies,
+  DefinitelyTruthy,
+  WITH_DEDUPE
 } from 'swr/_internal'
 
-const WITH_DEDUPE = { dedupe: true }
-
-type DefinitelyTruthy<T> = false extends T
-  ? never
-  : 0 extends T
-  ? never
-  : '' extends T
-  ? never
-  : null extends T
-  ? never
-  : undefined extends T
-  ? never
-  : T
-
 export const useSWRHandler = <Data = any, Error = any>(
   _key: Key,
   fetcher: Fetcher<Data> | null,