Escape the string during toJSON.

[humdrum]

What this PR does / why we need it:

• By the JSON spec. special characters must be escaped with a slash. (eg. newline => \n) (https://www.json.org/json-en.html)
• The toJSON in JSON Node makes JSON string without escaping now. So toJSON string can't be parsed.
  • eg. JSONTreeTextNode(value: "\n").toJSONString

{"type": "text", "value": "
"}

• I added toJSONString var in String Extension. And explicitly escape the string in toJSONString.
• I also apply a similar modification to the CRDTTree.

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Additional documentation:

Checklist:

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

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (4a63cca) 43.02% compared to head (effce02) 43.07%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #128      +/-   ##
==========================================
+ Coverage   43.02%   43.07%   +0.05%     
==========================================
  Files         104      104              
  Lines       22932    22958      +26     
==========================================
+ Hits         9866     9890      +24     
- Misses      13066    13068       +2     

Files	Coverage Δ
Tests/Unit/Document/CRDT/CRDTTreeTests.swift	100.00% <100.00%> (ø)
Tests/Unit/Document/JONSTreeTests.swift	100.00% <100.00%> (ø)
Sources/Document/CRDT/CRDTTree.swift	66.93% <66.66%> (ø)
Sources/Util/String+Extensions.swift	57.14% <77.77%> (+7.14%)	⬆️
Sources/Document/Json/JSONTree.swift	21.89% <37.50%> (ø)

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[myupage]

Works great. Thank you.

[hackerwins]

Thanks for your contribution.

@7hong13 @chacha912 @krapie
This should be applied to other SDKs.