FunctionNaming: allow anonymous functions

[t-kameyama]

Fixes #3054

[codecov]

Codecov Report

Merging #3055 into master will decrease coverage by 0.01%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##             master    #3055      +/-   ##
============================================
- Coverage     79.31%   79.30%   -0.02%     
- Complexity     2562     2563       +1     
============================================
  Files           432      432              
  Lines          7726     7726              
  Branches       1466     1466              
============================================
- Hits           6128     6127       -1     
  Misses          814      814              
- Partials        784      785       +1     

Impacted Files	Coverage Δ	Complexity Δ
...b/arturbosch/detekt/rules/naming/FunctionNaming.kt	91.66% <33.33%> (-4.17%)	11.00 <0.00> (+1.00)	⬇️

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