feat: add avoid-unnecessary-conditionals rule

[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:

Addresses https://github.com/dart-lang/linter/issues/3888 and flutter/devtools#4890

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
No Duplication information

[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: 532

• Total lines of source code: 9396

• Total classes: 402

• Average Cyclomatic Number per line of code: 0.37 / 2

• Average Source Lines of Code per method: 6

• Total tech debt: 1952.0 hours

• Found issues: 5 ⚠

[codecov]

Codecov Report

Merging #1094 (1ef030a) into master (7db92de) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    dart-lang/linter#1094      +/-   ##
==========================================
+ Coverage   84.97%   85.03%   +0.05%     
==========================================
  Files         335      337       +2     
  Lines        7361     7389      +28     
==========================================
+ Hits         6255     6283      +28     
  Misses       1106     1106              

Impacted Files	Coverage Δ
...c/analyzers/lint_analyzer/rules/rules_factory.dart	75.00% <ø> (ø)
...ditionals/avoid_unnecessary_conditionals_rule.dart	100.00% <100.00%> (ø)
...s_list/avoid_unnecessary_conditionals/visitor.dart	100.00% <100.00%> (ø)

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more