refactor: Replace _.isFunction(value) with typeof value === 'function'

[wingkwong]

Addresses: #7747

[codecov-commenter]

Codecov Report

Merging #7810 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #7810   +/-   ##
=======================================
  Coverage   88.18%   88.18%           
=======================================
  Files         245      245           
  Lines        9305     9305           
=======================================
  Hits         8206     8206           
  Misses       1099     1099           

Impacted Files	Coverage Δ
lib/classes/Variables.js	99.72% <ø> (ø)
lib/classes/PluginManager.js	96.67% <100.00%> (ø)

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[medikoo]

Thank you @wingkwong !