Add missing watchify dependency

[SimenB]

Summary
Currently a missing peer dep warning is printed, and karma start --watch fails.

Test plan
N/A

[codecov-io]

Codecov Report

Merging #4366 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4366   +/-   ##
=======================================
  Coverage   55.85%   55.85%           
=======================================
  Files         189      189           
  Lines        6383     6383           
  Branches        6        6           
=======================================
  Hits         3565     3565           
  Misses       2815     2815           
  Partials        3        3

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[github-actions]

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