feat: improve avoid unnecessary type assertions

[dkrutskikh]

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix
[ ] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

[codecov]

Codecov Report

Merging #560 (386acc1) into master (bd19b37) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #560      +/-   ##
==========================================
+ Coverage   85.37%   85.42%   +0.05%     
==========================================
  Files         222      222              
  Lines        4566     4570       +4     
==========================================
+ Hits         3898     3904       +6     
+ Misses        668      666       -2     

Impacted Files	Coverage Δ
...rtions/avoid_unnecessary_type_assertions_rule.dart	100.00% <100.00%> (ø)
...ist/avoid_unnecessary_type_assertions/visitor.dart	100.00% <100.00%> (ø)
...c/analyzers/lint_analyzer/rules/rules_factory.dart	93.93% <0.00%> (+3.03%)	⬆️

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