Add RealtimeSyncOff and refactor interface of SyncMode

[7hong13]

What this PR does / why we need it?

The SyncOff mode will allow users to maintain the watch stream functionality while disabling synchronization.

- 1️⃣ With the addition of the `RealtimeSyncOff` mode, the `SyncMode` has been simplified into four options. Usage is as follows:

// as-is
await client.pause(doc); // manual sync
await client.resume(doc); // realtime sync(pushpull)
client.pauseRemoteChanges(doc); // pushonly 
client.resumeRemoteChanges(doc); // pushpull 

// to-be
await client.changeSyncMode(doc, SyncMode.Manual);
await client.changeSyncMode(doc, SyncMode.Realtime); // Same as PushPull mode
await client.changeSyncMode(doc, SyncMode.RealtimePushOnly); 
await client.changeSyncMode(doc, SyncMode.RealtimeSyncOff); // Added RealtimeSyncOff mode 

- 2️⃣ In manual sync mode, the `sync()` function can now only be executed in pushpull mode.

await client.sync(doc, SyncMode.PushOnly) // Deprecated: Only pushpull synchronization is supported now.

- 3️⃣ The option to specify manual sync during attachment has been updated.

// as-is 
await client.attach(doc, { isRealtimeSync: false });

// to-be
await client.attach(doc, { syncMode: SyncMode.Manual });

Any background context you want to provide?

What are the relevant tickets?

• Add RealtimeSyncOff and refactor interface of SyncMode yorkie-js-sdk#772

Checklist

• Added relevant tests or not required
• Didn't break anything

[7hong13]

it seems like codecov keeps failing since it doesn't support tokenless uploading anymore.

• README
• Upload with v3 action fails because of missing token  codecov/codecov-action#1359

(cc. @hackerwins )