Fix ValShouldBeVar false positive inside unknown type

[themkat]

Attempt at fixing false positives for ValShouldBeVar when the assignment is inside an unknown type. This is the only logical solution I can think of at the moment. It basically assumes that if we can't resolve the type of the assignment, then it is from the inside of an unknown context (like a with-statement). Let me know if there is something I'm missing, and we can close/reject this PR...

Fixes #4736

[codecov]

Codecov Report

Merging #4820 (62f7683) into main (faa985d) will increase coverage by 0.03%.
The diff coverage is 50.00%.

@@             Coverage Diff              @@
##               main    #4820      +/-   ##
============================================
+ Coverage     84.71%   84.74%   +0.03%     
  Complexity     3431     3431              
============================================
  Files           491      491              
  Lines         11267    11270       +3     
  Branches       2075     2074       -1     
============================================
+ Hits           9545     9551       +6     
  Misses          673      673              
+ Partials       1049     1046       -3     

Impacted Files	Coverage Δ
...lab/arturbosch/detekt/rules/style/VarCouldBeVal.kt	81.55% <50.00%> (+1.16%)	⬆️
...turbosch/detekt/rules/complexity/NamedArguments.kt	83.33% <0.00%> (+1.19%)	⬆️
...lab/arturbosch/detekt/rules/style/UnusedImports.kt	89.87% <0.00%> (+2.53%)	⬆️

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