feat(markdown-it): support markdown-it-async integration

[antfu]

Integrate with https://github.com/antfu/markdown-it-async

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[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.27%. Comparing base (efce837) to head (8012810).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/markdown-it/src/async.ts	96.36%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #902      +/-   ##
==========================================
+ Coverage   94.25%   94.27%   +0.01%     
==========================================
  Files          89       91       +2     
  Lines        7415     7471      +56     
  Branches     1528     1536       +8     
==========================================
+ Hits         6989     7043      +54     
- Misses        417      419       +2     
  Partials        9        9              

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