Support concurrent formatting of Text

[humdrum]

What this PR does / why we need it:

• This is the Swift implementation of JS PR below.
  • Support concurrent formatting of Text yorkie-js-sdk#642
• Port every TC for Text from JS.
• toSortedJSON() of JSONText returns sorted attributes too.
• change var name "plainText" of JSONText to "toString" to match the name of JS

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: 11.92% and project coverage change: -0.90% ⚠️

Comparison is base (38ea3c2) 44.24% compared to head (84ed297) 43.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
- Coverage   44.24%   43.35%   -0.90%     
==========================================
  Files         105      105              
  Lines       22229    22888     +659     
==========================================
+ Hits         9836     9922      +86     
- Misses      12393    12966     +573     

Files Changed	Coverage Δ
Sources/API/Converter.swift	43.45% <0.00%> (-0.20%)	⬇️
Sources/API/V1/yorkie/v1/resources.pb.swift	6.83% <0.00%> (-0.03%)	⬇️
Tests/Integration/TextIntegrationTests.swift	0.00% <0.00%> (-1.39%)	⬇️
Sources/Document/CRDT/CRDTText.swift	88.99% <100.00%> (+2.90%)	⬆️
Sources/Document/CRDT/RGATreeSplit.swift	78.38% <100.00%> (+0.72%)	⬆️
Sources/Document/Json/JSONText.swift	66.88% <100.00%> (+6.48%)	⬆️
Sources/Document/Operation/StyleOperation.swift	53.65% <100.00%> (+1.15%)	⬆️
Tests/Unit/Document/DocumentTests.swift	98.58% <100.00%> (ø)
Tests/Unit/Document/JSONTextTest.swift	100.00% <100.00%> (ø)

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[hackerwins]

Thanks for your contribution.