Move contexts over to useXContext model

Fixes #4

src/lib/components/switch/SwitchGroup.svelte

@@ -2,12 +2,19 @@
   export interface StateDefinition {
     switchStore: Writable<HTMLButtonElement | null>;
   }
+
+  const SWITCH_CONTEXT_NAME = "SwitchContext";
+  export function useSwitchContext():
+    | Writable<StateDefinition | undefined>
+    | undefined {
+    return getContext(SWITCH_CONTEXT_NAME);
+  }
 </script>
 
 <script lang="ts">
   import DescriptionProvider from "./DescriptionProvider.svelte";
   import LabelProvider from "./LabelProvider.svelte";
-  import { setContext } from "svelte";
+  import { getContext, setContext } from "svelte";
   import { Writable, writable } from "svelte/store";
 
   let switchStore: Writable<HTMLButtonElement | null> = writable(null);