feat: add avoid-substring rule

[lsaudon]

Issue: #1201

• Read the contributing guidelines.
• Followed lint naming conventions
• Tests pass locally
• Updated the changelog
• Added the docs to this PR

Rule documentation

Replace s.substring(100,200) to s.characters.getRange(100,200).

As we know, using String.substring method will cause problems if you are having emojis in the string. 
There are many articles in the web, such as [substring does not support UTF-16](https://github.com/dart-lang/sdk/issues/35798).

Good example

s.characters.getRange(14, 15); 

Bad example

s.substring(14, 15); 

[codecov]

Codecov Report

Merging #1206 (9362821) into master (96001b5) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 9362821 differs from pull request most recent head 0c6df2f. Consider uploading reports for the commit 0c6df2f to get more accurate results

@@            Coverage Diff             @@
##           master    #1206      +/-   ##
==========================================
+ Coverage   86.21%   86.24%   +0.03%     
==========================================
  Files         359      361       +2     
  Lines        8133     8152      +19     
==========================================
+ Hits         7012     7031      +19     
  Misses       1121     1121              

Impacted Files	Coverage Δ
...c/analyzers/lint_analyzer/rules/rules_factory.dart	100.00% <ø> (ø)
...les_list/avoid_substring/avoid_substring_rule.dart	100.00% <100.00%> (ø)
...yzer/rules/rules_list/avoid_substring/visitor.dart	100.00% <100.00%> (ø)

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[incendial]

Thank you for the contribution. If you'd like to have a Teams version license for 6 months, let me know.