fix: avoid-global-state to support static fields

[vlkonoshenko]

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

[ ] Documentation update
[X] 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:

#643

What changes did you make? (Give an overview)

Added support static fields.

[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: 357

• Total lines of source code: 6386

• Total classes: 240

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

• Average Source Lines of Code per method: 6

• Total tech debt: 1034.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #644 (4938c92) into master (62e5602) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #644   +/-   ##
=======================================
  Coverage   86.15%   86.16%           
=======================================
  Files         235      235           
  Lines        5043     5046    +3     
=======================================
+ Hits         4345     4348    +3     
  Misses        698      698           

Impacted Files	Coverage Δ
...r/rules/rules_list/avoid_global_state/visitor.dart	100.00% <100.00%> (ø)

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