fix(types): MiddlewareAPI getState should have (this: void)

Motivation:
The `getState` function provided by `MiddlewareAPI` is not bound to any
object, and as such it can be used freely as in the examples.

The `MiddlewareAPI` type does not make this explicit, and typescript
linters can report this as problematic usage.

src/types/middleware.ts

@@ -2,7 +2,7 @@ import type { Dispatch } from './store'
 
 export interface MiddlewareAPI<D extends Dispatch = Dispatch, S = any> {
   dispatch: D
-  getState(): S
+  getState(this: void): S
 }
 
 /**