fix(minify): set the default value of esbuild's legalComments option to external

[sunnylost]

Fixes #17892

According to the issue, legal comments from files are split into a separate file by default. However, I have a few questions:

1. Should external be the default value of legalComments?

2. All comments are output to a file named LEGAL.txt, but should there be an option to specify the output path/filename?

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[sapphi-red]

Should external be the default value of legalComments?

I think linked is better.

All comments are output to a file named LEGAL.txt, but should there be an option to specify the output path/filename?

I think we don't need an option for that. I don't think there're any cases where you need to specify the filename.

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I wonder if we should add a new build.legalComments option instead of using esbuild.legalComments. I think users would like to control the behavior even if they are using terser for JS minifier or lightningcss for CSS minifier. Currently both terser and lightningcss doesn't have an option to extract the license comments though.

[sapphi-red]

I think we need to use name: 'LEGAL.txt' instead so that the file name includes a hash. That requires the res.code += '\n/*! For license information please see LEGAL.txt */' to be moved into generateBundle hook.

[sapphi-red]

I think we want to add /*! For license information please see LEGAL.txt */ to the CSS as well.

[sunnylost]

Sounds reasonable.