Add a new field Library.generatedByComment to support emitting 'generated by' comments.

[devoncarew]

• Add a new field Library.generatedByComment to support emitting 'generated by' comments.

This is an alternative to #439.

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[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:code_builder	4.9.0	ready to publish	v4.9.0

Documentation at https://github.com/dart-lang/ecosystem/wiki/Publishing-automation.

[srawlins]

Oh hey, I really like this idea. This sounds really slick to me.