Extract some common Yjs code into Vault class

src/vault.ts

@@ -0,0 +1,31 @@
+import type { LocalDocument } from './sync';
+import * as Y from 'yjs';
+
+export class Vault {
+  doc: LocalDocument;
+
+  constructor(doc: LocalDocument) {
+    this.doc = doc;
+  }
+
+  getDocInfo(id: string): DocInfo | null {
+    const docsEntry = this.getDocsMap().get(id);
+    if (!docsEntry) {
+      return null;
+    }
+    return {
+      title: docsEntry.get('title'),
+      tags: new Set(),
+    };
+  }
+
+  getDocsMap(): Y.Map<Y.Map<any>> {
+    const vaultMap = this.doc.doc.getMap();
+    return vaultMap.get('docs') as Y.Map<any>;
+  }
+}
+
+export interface DocInfo {
+  title: string;
+  tags: Set<string>;
+}