Complete redo of content indexing and querying (#517)

Complete redo of data store
Introduces live queries and live templates

cli/plug_test.ts

@@ -1,7 +1,7 @@
-import { index } from "$sb/silverbullet-syscall/mod.ts";
+import { datastore } from "$sb/syscalls.ts";
 
 export async function run() {
   console.log("Hello from plug_test.ts");
-  console.log(await index.queryPrefix(`tag:`));
+  await datastore.set(["plug_test"], "Hello");
   return "Hello";
 }