Rule 4.1.3 Try to use 'val' instead of 'var' for variable declaration

[orchestr7]

Adding simple rule that prohibits usage of var

What's done:

1. Adding rule and variable detection utility

[codecov]

Codecov Report

Merging #328 into master will increase coverage by 0.00%.
The diff coverage is 79.31%.

@@            Coverage Diff            @@
##             master     #328   +/-   ##
=========================================
  Coverage     82.82%   82.82%           
- Complexity     1127     1132    +5     
=========================================
  Files            54       56    +2     
  Lines          2800     2836   +36     
  Branches        893      908   +15     
=========================================
+ Hits           2319     2349   +30     
  Misses          162      162           
- Partials        319      325    +6     

Flag	Coverage Δ	Complexity Δ
#unittests	82.82% <79.31%> (+<0.01%)	1132.00 <5.00> (+5.00)

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Impacted Files	Coverage Δ	Complexity Δ
...at-rules/src/main/kotlin/generated/WarningNames.kt	0.00% <ø> (ø)	0.00 <0.00> (ø)
...n/kotlin/org/cqfn/diktat/ruleset/utils/PsiUtils.kt	41.93% <50.00%> (-12.24%)	0.00 <0.00> (ø)
...tlin/org/cqfn/diktat/ruleset/utils/AstNodeUtils.kt	82.65% <60.00%> (-0.60%)	0.00 <0.00> (ø)
...qfn/diktat/ruleset/utils/VariableSearchASTUtils.kt	81.25% <81.25%> (ø)	0.00 <0.00> (?)
...cqfn/diktat/ruleset/rules/ImmutableValNoVarRule.kt	85.71% <85.71%> (ø)	5.00 <5.00> (?)
...tlin/org/cqfn/diktat/ruleset/constants/Warnings.kt	96.87% <100.00%> (+0.03%)	9.00 <0.00> (ø)
...cqfn/diktat/ruleset/rules/DiktatRuleSetProvider.kt	97.82% <100.00%> (+0.04%)	3.00 <0.00> (ø)
...at/ruleset/rules/identifiers/LocalVariablesRule.kt	92.06% <100.00%> (ø)	32.00 <0.00> (ø)

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