Shorten how intersectors are added to the grid in the Intersector constructor

[Snuffleupagus]

Thanks to modern JavaScript features this code can be simplified a tiny bit.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.95%. Comparing base (d92db13) to head (4a5c455).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21200      +/-   ##
==========================================
+ Coverage   55.71%   55.95%   +0.24%     
==========================================
  Files         209      218       +9     
  Lines       57398    59063    +1665     
==========================================
+ Hits        31977    33048    +1071     
- Misses      25421    26015     +594     

Flag	Coverage Δ
fonttest	8.66% <ø> (?)
unittest	55.06% <100.00%> (?)
unittestcli	55.70% <100.00%> (-0.01%)	⬇️

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[timvandermeij]

r=me; thanks!