code shouldn't be stored as character

In R, there are more appropriate structures to store the _code_ but we keep storing this as a character.

This has some indirect negative consequences of unnecessary type conversions. Right now, if I call `quenv() |> within({})` then my `` is [converted to character](https://github.com/insightsengineering/teal.code/blob/3c1a37052c775a0d4466a8dbe852db2b909f6b31/R/qenv-eval_code.R#L94-L96) and then [back again to the code](https://github.com/insightsengineering/teal.code/blob/3c1a37052c775a0d4466a8dbe852db2b909f6b31/R/qenv-eval_code.R#L36). Such behaviour is currently being adapted in `teal.data` (`eval_code.teal_data_module`) so if changed - please change there as well.