Add toJS to return TreeNode of Tree

[humdrum]

What this PR does / why we need it:

• This is the Swift implementation of JS PR below.
  • Add toJS to return TreeNode of Tree yorkie-js-sdk#639

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

Patch coverage: 50.00% and project coverage change: -0.01% ⚠️

Comparison is base (c73041b) 44.24% compared to head (fd284cd) 44.24%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   44.24%   44.24%   -0.01%     
==========================================
  Files         105      105              
  Lines       22229    22239      +10     
==========================================
+ Hits         9836     9839       +3     
- Misses      12393    12400       +7     

Files Changed	Coverage Δ
Sources/Document/Json/JSONTree.swift	17.77% <0.00%> (-0.36%)	⬇️
Sources/Document/CRDT/CRDTTree.swift	66.40% <100.00%> (+0.15%)	⬆️

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[hackerwins]

Thanks for your contribution.