fix: copying multiple times only kept last undo CellExternalCopyManager

[ghiscoding]

• fix an issue where copying and pasting to multiple area only kept the last undo and reapplied it over and over and the cause was because the clipCommand was global to the class so it only kept the last undo, the fix is to make sure the clipCommand variable is local to the execution handler instead

[codecov]

Codecov Report

Merging #1075 (4b72929) into master (07efeb6) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1075   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          244       244           
  Lines        16564     16562    -2     
  Branches      5933      5933           
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- Hits         16564     16562    -2     

Files Changed	Coverage Δ
...mon/src/extensions/slickCellExternalCopyManager.ts	100.00% <100.00%> (ø)