feat(ferry_generator): add option to disable formatting

[Vild]

Adds a flag to the build.xml's options to prefix the generated code with // dart format off. I set the default value is true to not affect any existing output.

Do I need to make any other changes for this simple feature?

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[knaeckeKami]

Thanks and sorry for the long delay, i missed this.

Please document this feature in

docs/codegen.md, this will be used the generate the website.

[Vild]

Done :)