Add the toJS method to the ObjectProxy's handler

[chacha912]

What this PR does / why we need it?

Add the toJS method to the ObjectProxy's handler.
Previously, toJSON returns a JSON string.
It requires an extra step of JSON.parse to use JSON object.
After this PR, it is easier to use the JSON object with toJS method.

doc.subscribe((event) => {
  // as-is
  const rootObject = JSON.parse(doc.getRoot().toJSON());

  // to-be
  const rootObject = doc.getRoot().toJS();
});

Any background context you want to provide?

What are the relevant tickets?

Fixes #444

Checklist

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

[codecov]

Codecov Report

Merging #449 (14ba735) into main (9e1e776) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #449      +/-   ##
==========================================
+ Coverage   89.27%   89.30%   +0.02%     
==========================================
  Files          69       69              
  Lines        5296     5309      +13     
  Branches      530      531       +1     
==========================================
+ Hits         4728     4741      +13     
  Misses        385      385              
  Partials      183      183              

Impacted Files	Coverage Δ
src/document/document.ts	79.00% <ø> (ø)
src/document/json/object.ts	88.23% <100.00%> (+0.54%)	⬆️
test/integration/object_test.ts	100.00% <100.00%> (ø)
src/yorkie.ts	100.00% <0.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[hackerwins]

Thanks for your contribution.

What about adding test codes for this?

[hackerwins]

Thanks for your contribution.