feat(twoslash): expose core module, independent from twoslash

[antfu]

This allows to pass your custom twoslash handler.

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[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (be4106b) 96.87% compared to head (3774cd0) 96.88%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
packages/shikiji-twoslash/src/core.ts	95.10%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   96.87%   96.88%   +0.01%     
==========================================
  Files          44       45       +1     
  Lines        3587     3600      +13     
  Branches      477      478       +1     
==========================================
+ Hits         3475     3488      +13     
  Misses        112      112              

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