feat: add @checkbox-border-radius

[gherciu]

Add the possibility to control the checkbox border-radius using a separate less variable.

[中文版模板 / Chinese template]

🤔 This is a ...

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

🔗 Related issue link

No related issues it's a feature PR

💡 Background and solution

At the moment there is no possibility to separatelly control checkbox border-radius with less variables. It uses the base border-radius variable which is ok by default.... But a lot of peoples likes to make their components more round with a border e.g(10px) and all components look good but not the checkbox because it becomes fully Round/Circle because of it's small size. And this PR is meant to provide a way to control separately only the checkbox border-radius if is needed.

📝 Changelog

Language	Changelog
🇺🇸 English	Add @checkbox-border-radius less variable.
🇨🇳 Chinese	新增 Less 变量 @checkbox-border-radius。

☑️ 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
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

[afc163]

[github-actions]

[codecov]

Codecov Report

Merging #31360 (5dfe093) into feature (f45f735) will increase coverage by 1.14%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           feature   #31360      +/-   ##
===========================================
+ Coverage    98.81%   99.96%   +1.14%     
===========================================
  Files          363      400      +37     
  Lines         7280     7579     +299     
  Branches      1967     2138     +171     
===========================================
+ Hits          7194     7576     +382     
+ Misses          86        3      -83     

Impacted Files	Coverage Δ
components/_util/easings.ts	100.00% <ø> (ø)
components/_util/raf.ts	100.00% <ø> (ø)
components/_util/scrollTo.ts	100.00% <ø> (ø)
components/_util/type.ts	100.00% <ø> (ø)
components/calendar/generateCalendar.tsx	100.00% <ø> (+1.33%)	⬆️
components/calendar/index.tsx	100.00% <ø> (ø)
components/card/Grid.tsx	100.00% <ø> (ø)
components/_util/ActionButton.tsx	100.00% <100.00%> (ø)
components/_util/getScroll.tsx	100.00% <100.00%> (ø)
components/_util/hooks/useFlexGapSupport.ts	100.00% <100.00%> (ø)
... and 246 more

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