Use unknown rather than any for BaseController state

The BaseController state now uses `unknown` rather than `any` as the
type for state properties. `unknown` is more type-safe than `any` in
cases like this where we don't know what type to expect. See here for
details [1].

This was suggested by @rekmarks during review of #362 [2].

[1]: microsoft/TypeScript#24439
[2]: #362 (comment)

src/BaseControllerV2.ts

@@ -14,7 +14,7 @@ export type Listener<T> = (state: T, patches: Patch[]) => void;
 /**
  * Controller class that provides state management and subscriptions
  */
-export class BaseController<S extends Record<string, any>> {
+export class BaseController<S extends Record<string, unknown>> {
   private internalState: S;
 
   private internalListeners: Set<Listener<S>> = new Set();