fix(sheet): merge cell custom property

[Dushusir]

close #3346

What has changed?

cell.custom used to overwrite the entire thing, causing the original custom to be lost. Now it has been fixed to be similar to cell style, and needs to be merged when updating cell.custom (one-layer merge)

How to test?

1. Set new custom data to A1 univerAPI.getActiveWorkbook().getActiveSheet().getRange(0,0,1,1).setValue({custom:{a:1}})
2. Check cell data univer._univerInstanceService.getFocusedUnit()._snapshot
3. Add more custom data to A1 univerAPI.getActiveWorkbook().getActiveSheet().getRange(0,0,1,1).setValue({custom:{b:1}})
4. Check cell data
   

Pull Request Checklist

• Related tickets or issues have been linked in the PR description (or missing issue).
• Naming convention is followed (do please check it especially when you created new plugins, commands and resources).
• Unit tests have been added for the changes (if applicable).
• Breaking changes have been documented (or no breaking changes introduced in this PR).

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 30.21%. Comparing base (56a27a7) to head (cb6b6d0).
Report is 2 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #3365      +/-   ##
==========================================
+ Coverage   30.20%   30.21%   +0.01%     
==========================================
  Files        2109     2110       +1     
  Lines      110029   110047      +18     
  Branches    23922    23930       +8     
==========================================
+ Hits        33233    33251      +18     
  Misses      76796    76796              

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[hexf00]

reverted in #3431