These annotations make it clear to the reader of a JavaScript source
where the build process outputs to. These annotations can later be
integrated in a webpack configuration. This way there is one source of
truth.
The `build` folder is omitted from the paths, because a single JS file
shouldn't not be responsible of knowing where outputs in general will
end up at. A file only knows its output location relative to the
project.
Props adamsilverstein, herregroen, omarreiss, pento.
Fixes#44361.
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