feat(collapse): add 'header-extra' slot

[LYErin]

close #1046

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[codecov]

Codecov Report

Merging #1064 (422e443) into main (286e29d) will decrease coverage by 0.09%.
The diff coverage is 75.00%.

❗ Current head 422e443 differs from pull request most recent head cb64f3c. Consider uploading reports for the commit cb64f3c to get more accurate results

@@            Coverage Diff             @@
##             main    #1064      +/-   ##
==========================================
- Coverage   47.33%   47.23%   -0.10%     
==========================================
  Files         511      511              
  Lines       12638    12626      -12     
  Branches     3563     3555       -8     
==========================================
- Hits         5982     5964      -18     
- Misses       5639     5643       +4     
- Partials     1017     1019       +2     

Impacted Files	Coverage Δ
src/collapse/src/CollapseItem.tsx	80.00% <75.00%> (-1.58%)	⬇️
src/steps/src/Step.tsx	73.80% <0.00%> (-14.29%)	⬇️
src/_utils/vue/flatten.ts	56.25% <0.00%> (-4.87%)	⬇️
src/cascader/src/Cascader.tsx	17.74% <0.00%> (-4.10%)	⬇️
src/space/src/Space.tsx	100.00% <0.00%> (ø)
src/message/src/Message.tsx	70.58% <0.00%> (ø)
src/upload/src/interface.ts	100.00% <0.00%> (ø)
src/cascader/src/interface.ts	100.00% <0.00%> (ø)
src/code/src/styles/index.cssr.ts	100.00% <0.00%> (ø)
src/descriptions/src/Descriptions.tsx	87.03% <0.00%> (ø)
... and 3 more

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[07akioni]

这个是从那里继承的，只在 header main 作用的话应该不会影响这里吧

[LYErin]

那就改成固定font-size: 14？

[07akioni]

我说的是 font-weight