fix: key and value text should be selectable

[pionxzh]

closes #93

Problem

The text was selectable in V1 but not in V2.
This is because we didn't noticed that userselect: none was introduced by MUI accidentally.

Solution

Add back userselect: text CSS.
And we also improved the selectable range to only the content, which is different from react-json-view. (Check the #screenshots)

Screenshots

@textea/json-viewer

react-json-view

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[codecov]

Codecov Report

Base: 87.40% // Head: 87.36% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (6aa02b2) compared to base (7884a29).
Patch coverage: 73.33% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
- Coverage   87.40%   87.36%   -0.05%     
==========================================
  Files          18       18              
  Lines        1969     1978       +9     
  Branches      349      349              
==========================================
+ Hits         1721     1728       +7     
- Misses        248      250       +2     

Impacted Files	Coverage Δ
src/components/DataTypes/Object.tsx	86.34% <60.00%> (-0.50%)	⬇️
src/components/DataKeyPair.tsx	69.01% <100.00%> (+0.33%)	⬆️
src/components/DataTypeLabel.tsx	92.85% <100.00%> (+0.26%)	⬆️

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