fix: getCellFromPoint() should return row/cell -1 outside grid canvas

[ghiscoding]

• the 1st approach was to throw an error when outside the grid canvas but that caused multiple errors when using auto-scroll (which is normal to fall outside the canvas to let the auto-scroll do its job). So to avoid all these errors, I opted with -1 instead when outside the canvas.
• So why not use return null? That would require too much refactoring and potentially bring a breaking change to the users who expect the getCellFromPoint() function to always return an object structured as { cell: a, row: b } (where a and b are numbers), so returning -1 has less impact and is still identifiable by being outside the canvas

cc @pbower

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c5f6b85) 97.7% compared to head (857fb98) 98.5%.
Report is 9 commits behind head on master.

❗ Current head 857fb98 differs from pull request most recent head 1939cae. Consider uploading reports for the commit 1939cae to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1325     +/-   ##
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+ Coverage    97.7%   98.5%   +0.8%     
========================================
  Files         198     198             
  Lines       21285   21272     -13     
  Branches     7093    7095      +2     
========================================
+ Hits        20788   20937    +149     
+ Misses        497     335    -162     

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