Use markdown-extensions

[remcohaszing]

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

I don’t know why it was problematic earlier, but markdown-extensions has just been changes to ESM itself and now has type definitions.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (e72d91a) 100.00% compared to head (5f0c634) 100.00%.

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Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1195   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          885       874   -11     
=========================================
- Hits           885       874   -11     

Files Changed	Coverage Δ
packages/remark-cli/cli.js	100.00% <100.00%> (ø)

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[ChristianMurphy]

Thanks @remcohaszing!

[wooorm]

My hunch is that when ESM started in Node, it couldn’t load JSON, even from ESM -> CJS -> JSON, but not sure anymore

[wooorm]

Thanks, also for getting the types in there!

[wooorm]

I don’t know why it was problematic earlier,

I remembered: ESM can, as you know, import CJS. But, at least at the start, when using ESM here, it could not load JSON in that CJS. markdown-extensions was JSON, and failed.