Switch to using JSR for syscalls calls

deno.jsonc

@@ -16,6 +16,6 @@
     ]
   },
   "imports": {
-    "$sb/": "https://deno.land/x/silverbullet/plug-api/"
+    "@silverbulletmd/silverbullet": "jsr:@silverbulletmd/silverbullet@^0.9.0"
   }
 }

hello.ts

@@ -1,4 +1,4 @@
-import { editor } from "$sb/syscalls.ts";
+import { editor } from "@silverbulletmd/silverbullet/syscalls";
 
 export async function helloWorld() {
   await editor.flashNotification("Hello world!");