refactor: move getLatestSelection to GridSelectionMouseHandler

[ericlln]

After merging in #2407, I thought a bit more about my implementation and decided to refactor it (before picking this back to v0.85)

It makes more sense for getLatestSelection to live in GridSelectionMouseHandler since its logic also depends on the implementation of onContextMenu in this class, and this will make it more straightforward for other mouse handlers that might want to use this method to get the Grid's most recent selected ranges.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 46.84%. Comparing base (c43f306) to head (a144891).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...d/src/mousehandlers/IrisGridContextMenuHandler.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2412   +/-   ##
=======================================
  Coverage   46.84%   46.84%           
=======================================
  Files         711      711           
  Lines       39316    39316           
  Branches     9825     9825           
=======================================
  Hits        18416    18416           
  Misses      20889    20889           
  Partials       11       11           

Flag	Coverage Δ
unit	46.84% <83.33%> (ø)

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