feat: introduce @shikijs/themes and @shikijs/langs packages

[antfu]

This PR moves langs and themes from shiki/langs/*.mjs to @shikijs/langs/* for better compsoable.

This is also one step towards #878

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.21%. Comparing base (c3524b2) to head (981bd58).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #879   +/-   ##
=======================================
  Coverage   95.21%   95.21%           
=======================================
  Files          83       83           
  Lines        7058     7058           
  Branches     1460     1460           
=======================================
  Hits         6720     6720           
  Misses        330      330           
  Partials        8        8           

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