Fix some more type errors and turn on TS strict mode

Possible now that svelte2tsx has fixed a few bugs!

src/lib/components/description/Description.svelte

@@ -10,7 +10,7 @@
     );
   }
 
-  onMount(() => $contextStore.register(id));
+  onMount(() => $contextStore?.register(id));
 </script>
 
 <p {...$$restProps} {...$contextStore?.props} {id}>

src/lib/components/dialog/Dialog.svelte

@@ -66,7 +66,7 @@
   $: {
     open =
       open === undefined && openClosedState !== undefined
-        ? match($openClosedState, {
+        ? match($openClosedState!, {
             [State.Open]: true,
             [State.Closed]: false,
           })

src/lib/components/label/Label.svelte

@@ -11,9 +11,9 @@
     );
   }
 
-  onMount(() => $contextStore.register(id));
+  onMount(() => $contextStore!.register(id));
 
-  let allProps = { ...$$restProps, ...$contextStore.props, id };
+  let allProps = { ...$$restProps, ...$contextStore!.props, id } as any;
   if (passive) delete allProps["onClick"];
 </script>