Fix to operate in consideration of grapheme cluster in Text

[hackerwins]

What this PR does / why we need it?

Fix to operate in consideration of grapheme cluster in Text.

Any background context you want to provide?

#167 (comment)

What are the relevant tickets?

Fixes #167
Fixes yorkie-team/yorkie#110

Checklist

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

[codecov]

Codecov Report

Merging #169 (bc49320) into main (2ffbba3) will increase coverage by 0.08%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##             main     #169      +/-   ##
==========================================
+ Coverage   78.60%   78.68%   +0.08%     
==========================================
  Files          48       48              
  Lines        2725     2740      +15     
  Branches      446      447       +1     
==========================================
+ Hits         2142     2156      +14     
- Misses        415      416       +1     
  Partials      168      168              

Impacted Files	Coverage Δ
src/document/json/plain_text.ts	78.68% <91.66%> (+3.17%)	⬆️
src/api/converter.ts	81.57% <100.00%> (ø)
src/document/json/rich_text.ts	65.00% <100.00%> (+1.08%)	⬆️

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[hackerwins]

I had Agent calculate text in UTF-16 code units in the same way as in JavaScript.
yorkie-team/yorkie#165