Remove Client.subscribe

[chacha912]

What this PR does / why we need it?

In this PR, Client.subscribe() was removed, and topics connection and sync were added to document.subscribe().

Any background context you want to provide?

What are the relevant tickets?

Fixes #788, #771

Checklist

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

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 80.28%. Comparing base (12522ef) to head (e146411).
Report is 1 commits behind head on main.

Files	Patch %	Lines
src/document/document.ts	68.18%	6 Missing and 1 partial ⚠️
src/devtools/index.ts	0.00%	3 Missing ⚠️
src/client/client.ts	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #789      +/-   ##
==========================================
- Coverage   80.46%   80.28%   -0.18%     
==========================================
  Files          59       59              
  Lines        4432     4439       +7     
  Branches      890      898       +8     
==========================================
- Hits         3566     3564       -2     
- Misses        604      612       +8     
- Partials      262      263       +1     

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[hackerwins]

Thanks for your contribution. 👍

[hackerwins]

@chacha912 It would be good to update the documentation for the next release. 🙏
https://yorkie.dev/docs/js-sdk#subscribing-to-client-events