implement JsonPrimitive

[skhugh]

What this PR does / why we need it?

Any background context you want to provide?

implements JsonPrimitive to make it easier to handle elements in JsonObject and JsonArray.
also renames Primitive to CrdtPrimitive to prevent confusion.

What are the relevant tickets?

Fixes #

Checklist

• Added relevant tests or not required
• Didn't break anything

[codecov]

Codecov Report

Merging #26 (6a42e65) into main (f1c5320) will decrease coverage by 0.91%.
The diff coverage is 21.87%.

@@             Coverage Diff              @@
##               main      #26      +/-   ##
============================================
- Coverage     46.49%   45.57%   -0.92%     
  Complexity      191      191              
============================================
  Files            31       32       +1     
  Lines           942      961      +19     
  Branches        130      132       +2     
============================================
  Hits            438      438              
- Misses          443      462      +19     
  Partials         61       61              

Impacted Files	Coverage Δ
...ain/kotlin/dev/yorkie/document/crdt/CrdtCounter.kt	88.37% <ø> (ø)
.../main/kotlin/dev/yorkie/document/json/JsonArray.kt	0.00% <0.00%> (ø)
...ain/kotlin/dev/yorkie/document/json/JsonElement.kt	0.00% <0.00%> (ø)
...main/kotlin/dev/yorkie/document/json/JsonObject.kt	0.00% <0.00%> (ø)
...n/kotlin/dev/yorkie/document/json/JsonPrimitive.kt	0.00% <0.00%> (ø)
...dev/yorkie/document/operation/IncreaseOperation.kt	0.00% <0.00%> (ø)
...n/kotlin/dev/yorkie/document/crdt/CrdtPrimitive.kt	79.16% <100.00%> (ø)
...ain/kotlin/dev/yorkie/document/crdt/RgaTreeList.kt	77.22% <100.00%> (ø)

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