Implement yorkie.Tree for text editors using tree model

[humdrum]

What this PR does / why we need it:

Implemented up to version 0.4.4 of JS.

• Implement yorkie.Tree for text editors using tree model yorkie-js-sdk#507
• Apply garbage collection to tree yorkie-js-sdk#550
• Use TreeRangeStruct to represent tree selection yorkie-js-sdk#561
• Cleanup of test-related terminology  yorkie-js-sdk#562

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

Merging #90 (6d84ab0) into main (e02ff33) will decrease coverage by 2.46%.
The diff coverage is 45.27%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
- Coverage   56.05%   53.60%   -2.46%     
==========================================
  Files          92      102      +10     
  Lines       14695    18483    +3788     
==========================================
+ Hits         8238     9907    +1669     
- Misses       6457     8576    +2119     

Files Changed	Coverage Δ
Sources/API/V1/yorkie/v1/resources.pb.swift	6.24% <0.00%> (-1.01%)	⬇️
Sources/API/V1/yorkie/v1/yorkie.grpc.swift	0.00% <ø> (ø)
Sources/API/V1/yorkie/v1/yorkie.pb.swift	0.00% <0.00%> (ø)
Sources/Core/Client.swift	0.60% <0.00%> (-0.03%)	⬇️
Sources/Document/CRDT/CRDTElement.swift	92.30% <ø> (ø)
Sources/Document/Json/ObjectDataHandler.swift	0.00% <0.00%> (ø)
Sources/Document/Operation/IncreaseOperation.swift	73.80% <0.00%> (ø)
Sources/Document/Operation/Operation.swift	39.43% <0.00%> (-21.44%)	⬇️
Sources/Document/Operation/SelectOperation.swift	46.34% <0.00%> (ø)
Sources/Document/Operation/StyleOperation.swift	52.50% <0.00%> (ø)
... and 54 more