feat(checkbox): support indeterminate status and slot

[chouchouji]

Checklist

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• add indeterminate status
• Tests have been added / updated (or snapshots)

Change information

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Name	Status	Preview	Comments	Updated (UTC)
varlet	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 24, 2023 9:47am

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[codecov]

Codecov Report

Merging #1159 (674d949) into dev (56cdff8) will increase coverage by 0.00%.
Report is 8 commits behind head on dev.
The diff coverage is 95.00%.

❗ Current head 674d949 differs from pull request most recent head c5fb262. Consider uploading reports for the commit c5fb262 to get more accurate results

@@           Coverage Diff           @@
##              dev    #1159   +/-   ##
=======================================
  Coverage   95.70%   95.71%           
=======================================
  Files         337      337           
  Lines       22098    22081   -17     
  Branches     2175     2162   -13     
=======================================
- Hits        21149    21134   -15     
+ Misses        898      896    -2     
  Partials       51       51           

Files Changed	Coverage Δ
packages/varlet-ui/src/checkbox/Checkbox.vue	96.47% <93.75%> (+0.17%)	⬆️
packages/varlet-ui/src/checkbox/props.ts	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes

[chouchouji]

How about adding an indeterminate slot to provide more flexibility？
like this

@haoziqaq