fix(typings): fn is assigning to where

[SimonSchick]

Pull Request check-list

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Description of change

Added Fn to the type union to allow performing model.findOne({ where: fn(...) }) which is currently supported by sequelize.

[codecov]

Codecov Report

Merging #12040 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12040   +/-   ##
=======================================
  Coverage   96.24%   96.24%           
=======================================
  Files          95       95           
  Lines        9215     9215           
=======================================
  Hits         8869     8869           
  Misses        346      346           

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[SimonSchick]

I think traves stumpled?