Docs: Add file doc @output annotations.

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.

Built from https://develop.svn.wordpress.org/trunk@43347


git-svn-id: http://core.svn.wordpress.org/trunk@43175 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/shortcode.js

@@ -1,4 +1,8 @@
-// Utility functions for parsing and handling shortcodes in JavaScript.
+/**
+ * Utility functions for parsing and handling shortcodes in JavaScript.
+ *
+ * @output wp-includes/js/shortcode.js
+ */
 
 /**
  * Ensure the global `wp` object exists.