SimpleEditCounter: add creation count

[agoujot]

Have checked performance: after merging with the regular edits subquery, shouldn't be any slower.

Bug: T229581

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.11%. Comparing base (4b8eb28) to head (ed74c74).
Report is 117 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #528      +/-   ##
============================================
- Coverage     71.30%   71.11%   -0.19%     
- Complexity     1275     1292      +17     
============================================
  Files            46       46              
  Lines          3792     3878      +86     
============================================
+ Hits           2704     2758      +54     
- Misses         1088     1120      +32     

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[agoujot]

FYI, the countsOnly option in pages is #550.