feat: add prefer-async-await rule

[incendial]

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

[ ] Documentation update
[ ] Bug fix
[X] New rule
[ ] Changes an existing rule
[ ] Add autofixing to a rule
[ ] Add a CLI option
[ ] Add something to the core
[ ] Other, please explain:

[incendial]

@roman-petrov could you please take a look too? Maybe I missed some cases.

[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: 387

• Total lines of source code: 6896

• Total classes: 259

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

• Average Source Lines of Code per method: 6

• Total tech debt: 1126.0 hours

• Found issues: 6 ⚠

[codecov]

Codecov Report

Merging #686 (35d5b7b) into master (6ea082e) will increase coverage by 0.02%.
The diff coverage is 94.73%.

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
+ Coverage   86.30%   86.33%   +0.02%     
==========================================
  Files         253      255       +2     
  Lines        5412     5431      +19     
==========================================
+ Hits         4671     4689      +18     
- Misses        741      742       +1     

Impacted Files	Coverage Δ
...c/analyzers/lint_analyzer/rules/rules_factory.dart	94.66% <0.00%> (-1.28%)	⬇️
...st/prefer_async_await/prefer_async_await_rule.dart	100.00% <100.00%> (ø)
...r/rules/rules_list/prefer_async_await/visitor.dart	100.00% <100.00%> (ø)

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[roman-petrov]

@incendial this looks promising! I am a bit ill and surely will take closer look in a couple of days.

[incendial]

@incendial this looks promising! I am a bit ill and surely will take closer look in a couple of days.

Sure, get well soon!

[dkrutskikh]

@incendial please set as false

[incendial]

Fixed