feat: introduce new code diagnostics prefer-single-widget-per-file

[dkrutskikh]

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:

What changes did you make? (Give an overview)

Is there anything you'd like reviewers to focus on?

[codecov-commenter]

Codecov Report

Merging #406 (836b4e7) into master (bc56884) will increase coverage by 0.07%.
The diff coverage is 92.30%.

❗ Current head 836b4e7 differs from pull request most recent head 45960e1. Consider uploading reports for the commit 45960e1 to get more accurate results

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
+ Coverage   81.04%   81.11%   +0.07%     
==========================================
  Files         166      169       +3     
  Lines        3766     3792      +26     
==========================================
+ Hits         3052     3076      +24     
- Misses        714      716       +2     

Impacted Files	Coverage Δ
...c/analyzers/lint_analyzer/rules/rules_factory.dart	91.83% <0.00%> (-3.91%)	⬇️
...s_list/member_ordering_extended/config_parser.dart	100.00% <ø> (ø)
...t/prefer_single_widget_per_file/config_parser.dart	100.00% <100.00%> (ø)
...widget_per_file/prefer_single_widget_per_file.dart	100.00% <100.00%> (ø)
...es_list/prefer_single_widget_per_file/visitor.dart	100.00% <100.00%> (ø)

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