Add toJSONString to JSONTreeNode

[humdrum]

What this PR does / why we need it:

• Add toJSONString to JSONTreeNode, so The user can get the changes by operations.

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: 11 lines in your changes are missing coverage. Please review.

Comparison is base (d80ad48) 42.98% compared to head (d15801d) 43.02%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #126      +/-   ##
==========================================
+ Coverage   42.98%   43.02%   +0.04%     
==========================================
  Files         103      104       +1     
  Lines       22840    22932      +92     
==========================================
+ Hits         9817     9866      +49     
- Misses      13023    13066      +43     

Files	Coverage Δ
Tests/Unit/Document/JONSTreeTests.swift	100.00% <100.00%> (ø)
Sources/Document/Json/JSONTree.swift	21.89% <70.27%> (+4.78%)	⬆️

... and 3 files with indirect coverage changes

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[hackerwins]

Thanks for your contribution.

[myupage]

Thank you