fix: support index expressions for no-magic-number rule

[incendial]

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:

[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: 415

• Total lines of source code: 7342

• Total classes: 283

• Average Cyclomatic Number per line of code: 0.34

• Average Source Lines of Code per method: 6

• Total tech debt: 1230.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #755 (bbab151) into master (6ae2211) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head bbab151 differs from pull request most recent head 11e2254. Consider uploading reports for the commit 11e2254 to get more accurate results

@@           Coverage Diff           @@
##           master     #755   +/-   ##
=======================================
  Coverage   86.90%   86.90%           
=======================================
  Files         273      273           
  Lines        5788     5790    +2     
=======================================
+ Hits         5030     5032    +2     
  Misses        758      758           

Impacted Files	Coverage Δ
...les_list/no_magic_number/no_magic_number_rule.dart	100.00% <100.00%> (ø)

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