feat: use official eslint-plugin-markdown package

[JounQin]

No description provided.

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[codecov-io]

Codecov Report

Merging #304 (31551b4) into master (7b8e7bc) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #304   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        21    -1     
  Lines          519       435   -84     
  Branches       101        86   -15     
=========================================
- Hits           519       435   -84     

Impacted Files	Coverage Δ
packages/eslint-plugin-mdx/src/processors/index.ts	100.00% <100.00%> (ø)
...ackages/eslint-plugin-mdx/src/processors/remark.ts	100.00% <100.00%> (ø)

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