feat(module: flex): add flex component

[ElderJames]

🤔 This is a ...

• New feature
• Bug fix
• Site / documentation update
• Demo update
• Component style update
• Bundle size optimization
• Performance optimization
• Refactoring
• Code style optimization
• Test Case
• Branch merge
• Other (about what?)

🔗 Related issue link

💡 Background and solution

📝 Changelog

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🇨🇳 Chinese

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• Changelog is provided or not needed

[github-actions]

[codecov]

Codecov Report

Attention: 38 lines in your changes are missing coverage. Please review.

Comparison is base (6a0bb0b) 44.66% compared to head (87efa11) 1.53%.

Files	Patch %	Lines
components/flex/Flex.razor.cs	0.00%	35 Missing ⚠️
components/core/Helpers/CssStyleBuilder.cs	0.00%	2 Missing ⚠️
components/core/CssSizeLength.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           feature   #3547       +/-   ##
===========================================
- Coverage    44.66%   1.53%   -43.14%     
===========================================
  Files          571     572        +1     
  Lines        27180   27149       -31     
  Branches      5605    5585       -20     
===========================================
- Hits         12141     417    -11724     
- Misses       14014   26692    +12678     
+ Partials      1025      40      -985     

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