Sync engine (#298)

Fixes #261

plug-api/silverbullet-syscall/mod.ts

@@ -3,6 +3,7 @@ export * as editor from "./editor.ts";
 export * as index from "./index.ts";
 export * as markdown from "./markdown.ts";
 export * as sandbox from "./sandbox.ts";
-export * as space from "./space.ts";
+export { default as space } from "./space.ts";
 export * as system from "./system.ts";
 export * as collab from "./collab.ts";
+export * as sync from "./sync.ts";