feat: get pending ops from current txn as json

[YngNs]

resolve #664

Example

  const doc = new LoroDoc();
  doc.setPeerId("1");
  expect(doc.getUncommittedOpsAsJson()).toBeUndefined();
  const text = doc.getText("text");
  text.insert(0, "Hello");
  const pendingOps = doc.getUncommittedOpsAsJson()
  expect(pendingOps).toBeDefined();
  expect(JSON.stringify(pendingOps)).toContain("insert");
  expect(JSON.stringify(pendingOps)).toContain("Hello");
  expect(pendingOps).toEqual({
    "peers": undefined,
    "schema_version": 1,
    "start_version": {},
    changes: [{
      "id": "0@1",
      "deps": [],
      "msg": undefined,
      "lamport": 0,
      "ops": [
        {
          "container": "cid:root-text:Text",
          "counter": 0,
          "content": {
            "type": "insert",
            "pos": 0,
            "text": "Hello"
          }
        }
      ],
      "timestamp": 0,
    }],
  })