Fix panel splitting

[erri120]

Fixes the panel splitting algorithm to respect the joined panels, depending on the orientation.

Related to #674 and 791.

[codecov-commenter]

Codecov Report

Merging #813 (7327b11) into main (17bf70e) will increase coverage by 61.45%.
The diff coverage is 92.69%.

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Additional details and impacted files

@@            Coverage Diff            @@
##           main     #813       +/-   ##
=========================================
+ Coverage      0   61.45%   +61.45%     
=========================================
  Files         0      618      +618     
  Lines         0    19028    +19028     
  Branches      0     1388     +1388     
=========================================
+ Hits          0    11693    +11693     
- Misses        0     6989     +6989     
- Partials      0      346      +346     

Flag	Coverage Δ
Linux	60.79% <92.69%> (?)
Windows	60.72% <92.69%> (?)
clean_environment_tests	61.43% <92.69%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/NexusMods.App.UI/WorkspaceSystem/GridUtils.cs	94.80% <100.00%> (ø)
...spaceSystem/WorkspaceGridState.ColumnEnumerator.cs	100.00% <100.00%> (ø)
...orkspaceSystem/WorkspaceGridState.RowEnumerator.cs	100.00% <100.00%> (ø)
...stem/WorkspaceGridState.AdjacentPanelEnumerator.cs	92.30% <92.30%> (ø)
...sMods.App.UI/WorkspaceSystem/WorkspaceGridState.cs	63.63% <78.26%> (ø)
...UI/WorkspaceSystem/Workspace/WorkspaceViewModel.cs	0.00% <0.00%> (ø)

... and 612 files with indirect coverage changes