fix: copyAssets

[peaceiris]

• deps: Add shelljs @types/shelljs
• feat: use shelljs.cp instead of actions/core/io/cp (io.cp)

https://github.com/shelljs/shelljs
https://www.npmjs.com/package/@types/shelljs

Related to:

• Weird nesting of content folders in HUGO build #507 keep_files creates subdir instead of overwriting contents of same-name dir #509
• Can't overwrite files on the branch with the action #390
• New Option destination_dir for deploying to a subdirectory #324
• inconsistent handling of symbolic links in publish_dir #103

TODO:

• remove build assets

[codecov]

Codecov Report

Merging #512 into main will increase coverage by 1.13%.
The diff coverage is 10.00%.

@@            Coverage Diff             @@
##             main     #512      +/-   ##
==========================================
+ Coverage   52.17%   53.30%   +1.13%     
==========================================
  Files           4        4              
  Lines         276      272       -4     
  Branches       47       47              
==========================================
+ Hits          144      145       +1     
+ Misses        131      126       -5     
  Partials        1        1              

Impacted Files	Coverage Δ
src/git-utils.ts	35.60% <10.00%> (+1.78%)	⬆️

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