feat: add static code diagnostics avoid-unnecessary-type-assertions

[vlkonoshenko]

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

[ ] Documentation update
[ ] Bug fix
[X] 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 #549 (5f997c9) into master (a9bef96) will increase coverage by 0.14%.
The diff coverage is 96.55%.

❗ Current head 5f997c9 differs from pull request most recent head cfb0b5d. Consider uploading reports for the commit cfb0b5d to get more accurate results

@@            Coverage Diff             @@
##           master     #549      +/-   ##
==========================================
+ Coverage   85.06%   85.20%   +0.14%     
==========================================
  Files         217      219       +2     
  Lines        4472     4530      +58     
==========================================
+ Hits         3804     3860      +56     
- Misses        668      670       +2     

Impacted Files	Coverage Δ
...c/analyzers/lint_analyzer/rules/rules_factory.dart	88.88% <0.00%> (-2.92%)	⬇️
..._assertions/avoid_unnecessary_type_assertions.dart	100.00% <100.00%> (ø)
...ist/avoid_unnecessary_type_assertions/visitor.dart	100.00% <100.00%> (ø)

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