add remark-footnotes-extra

[miaobuao]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and discussions and couldn’t find anything or linked relevant results below
• I made sure the docs are up to date
• I included tests (or that’s not needed)

Description of changes

Add a plugin: remark-footnotes-extra

How to use?

hello world^[reference]

acts like:

hello world [^1]

[^1]: reference

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (a185821) to head (3c7b773).
Report is 27 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1390   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          142       138    -4     
=========================================
- Hits           142       138    -4     

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[wooorm]

Thanks!