extend internal API for code generation to support assignments

[TSchiefer]

we need it (at least) for dm_paste() and future dm_deconstruct(). Might just work with using a list individual code blocks and write a function for adding the assignment.

see: https://github.com/cynkra/dm/pull/1095#pullrequestreview-1009875299

(not sure about milestone, but it doesn't seem relevant to me for the ui-milestone)