Rename and split ValidateConfig files

[BraisGabin]

The important part of the old file ValidatableConfiguration.kt wasn't the interface. It was the function validateConfig. And for that reason the tests were difficult to find.

With this PR I move the interface ValidatableConfiguration to its own file (it's not THAT related with validateConfig. And I create the file ValidateConfig.kt for the funcition validateConfig and its helpers. I rename the tests file to ValidateConfigSpec.kt to make it easier to find.

[codecov]

Codecov Report

Merging #4105 (6421e90) into main (e0aed91) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main    #4105   +/-   ##
=========================================
  Coverage     83.58%   83.58%           
  Complexity     3187     3187           
=========================================
  Files           459      459           
  Lines          9101     9101           
  Branches       1772     1772           
=========================================
  Hits           7607     7607           
  Misses          561      561           
  Partials        933      933           

Impacted Files	Coverage Δ
...ab/arturbosch/detekt/core/config/ValidateConfig.kt	100.00% <100.00%> (ø)

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