Clean up store types & initial values

Treating these as Readable for better safety from consuming code

Fixes #10

src/lib/internal/StackContextProvider.svelte

@@ -9,13 +9,13 @@
 
 <script lang="ts">
   import { getContext, onDestroy, setContext } from "svelte";
-  import { writable, Writable } from "svelte/store";
+  import { Readable, writable, Writable } from "svelte/store";
   type OnUpdate = (message: StackMessage, element: HTMLElement) => void;
 
   export let onUpdate: OnUpdate | undefined;
   export let element: HTMLElement | null;
 
-  let parentUpdateStore: Writable<OnUpdate> | undefined =
+  let parentUpdateStore: Readable<OnUpdate> | undefined =
     getContext(STACK_CONTEXT_NAME);
   let notifyStore: Writable<OnUpdate> = writable(() => {});
   setContext(STACK_CONTEXT_NAME, notifyStore);

src/lib/internal/open-closed.ts

@@ -1,5 +1,5 @@
 import { getContext, setContext } from "svelte";
-import type { Writable } from "svelte/store";
+import type { Readable, Writable } from "svelte/store";
 
 export enum State {
   Open,
@@ -11,7 +11,7 @@ export function hasOpenClosed() {
   return useOpenClosed() !== undefined;
 }
 
-export function useOpenClosed(): Writable<State> | undefined {
+export function useOpenClosed(): Readable<State> | undefined {
   return getContext(OPEN_CLOSED_CONTEXT_NAME);
 }