Extract some common Yjs code into Vault class

src/sync.ts

@@ -2,6 +2,7 @@ import _ from 'lodash';
 import * as Y from 'yjs';
 import { openDB } from 'idb';
 import { generateId } from './util';
+import { Vault } from './vault';
 
 const DB_NAME = 'synced-docs';
 const LOCAL_UPDATE_STORE = 'local-updates';
@@ -95,12 +96,12 @@ async function getOrCreateVaultDoc() {
   if (!vaultMap.has('docs')) {
     vaultMap.set('docs', new Y.Map());
   }
-  return vault;
+  return new Vault(vault);
 }
 
 export const defaultVault = await getOrCreateVaultDoc();
 
-const docsMap = defaultVault.doc.getMap().get('docs') as Y.Map<any>;
+const docsMap = defaultVault.getDocsMap();
 if (docsMap.size === 0) {
   console.log('Running vault migration...');
   for (const oldKey of ['foo', 'bar', 'baz']) {