Allow users to pass multi nodes when calling Tree.edit

[JOOHOJANG]

What this PR does / why we need it:

• allow users to pass multi tree nodes when edit
  • change parameter type *TreeNode => []*TreeNode
    • modify json/tree, crdt/tree and converter
• remove duplicate code
• update TC

FYI

• most of the lines that have changed are tc,
• please review
  Which issue(s) this PR fixes:

Fixes #561

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

Merging #579 (d1f5991) into main (2b15ac5) will decrease coverage by 0.10%.
The diff coverage is 35.13%.

@@            Coverage Diff             @@
##             main     #579      +/-   ##
==========================================
- Coverage   51.50%   51.40%   -0.10%     
==========================================
  Files          67       67              
  Lines        6932     6951      +19     
==========================================
+ Hits         3570     3573       +3     
- Misses       2891     2907      +16     
  Partials      471      471              

Impacted Files	Coverage Δ
api/converter/from_pb.go	48.20% <0.00%> (-0.75%)	⬇️
api/converter/to_bytes.go	68.58% <0.00%> (-3.71%)	⬇️
api/converter/to_pb.go	57.27% <0.00%> (ø)
pkg/document/crdt/tree.go	53.68% <72.22%> (+0.43%)	⬆️

[hackerwins]

Overall looks good.
I left a comment about style.

[hackerwins]

Thanks for your contribution.