remark-stringify: fix types to disallow options.extensions

[JounQin]

Initial checklist

• I read the support docs
• I read the contributing guide
• I agree to follow the code of conduct
• I searched issues and couldn’t find anything (or linked relevant results below)
• If applicable, I’ve added docs and tests

Description of changes

As title

[codecov-commenter]

Codecov Report

Merging #874 (c33ec4e) into main (d0d6f29) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main      #874   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          901       905    +4     
=========================================
+ Hits           901       905    +4     

Impacted Files	Coverage Δ
packages/remark-stringify/lib/index.js	100.00% <100.00%> (ø)

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[github-actions]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.

[wooorm]

released!