chore(core,shiki): add package.json export

[NWYLZW]

Description

Export package.json, add exports filed of package.json.
Can't find shiki/package.json, @shikijs/core/package.json in Node.js.

Linked Issues

#639

Additional context

[netlify]

✅ Deploy Preview for shiki-next ready!

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[netlify]

✅ Deploy Preview for shiki-matsu ready!

Name	Link
🔨 Latest commit	106983a
🔍 Latest deploy log	https://app.netlify.com/sites/shiki-matsu/deploys/66011514977dd300086f0e25
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.31%. Comparing base (ec2bffa) to head (106983a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #640   +/-   ##
=======================================
  Coverage   96.31%   96.31%           
=======================================
  Files          70       70           
  Lines        5699     5699           
  Branches      756      756           
=======================================
  Hits         5489     5489           
  Misses        205      205           
  Partials        5        5           

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