feat: make avoid-unnecessary-type-assertions handle is! checks

[incendial]

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:

#889

[github-actions]

Dart Code Metrics unused files report of dart_code_metrics. ✅

Summary

• Scanned package folders: bin, example, lib
• No unused files found! ✅

[github-actions]

Dart Code Metrics analyze report of dart_code_metrics. ✅

Summary

• Scanned folders: bin, example, lib, test

• Total scanned files: 466

• Total lines of source code: 8354

• Total classes: 324

• Average Cyclomatic Number per line of code: 0.35 / 1

• Average Source Lines of Code per method: 6

• Total tech debt: 1814.0 hours

• Found issues: 6 ⚠

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[codecov]

Codecov Report

Merging #995 (4df03d2) into master (20a42e5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #995   +/-   ##
=======================================
  Coverage   87.80%   87.81%           
=======================================
  Files         309      309           
  Lines        6519     6523    +4     
=======================================
+ Hits         5724     5728    +4     
  Misses        795      795           

Impacted Files	Coverage Δ
...ist/avoid_unnecessary_type_assertions/visitor.dart	100.00% <100.00%> (ø)

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