datanames slot in teal_transform (enhanced teal.data::datanames())

[gogonzo]

Done for insightsengineering/teal.data#330 where I enhanced teal.data::datanames() function to return ls(data@env) when @datanames is not specified. In the long term I suggest @datanames field to be discontinued.

[m7pr]

@gogonzo I agree that maintaining teal_data@datanames, modules$datanames and ls(get_env(teal_data)) became problematic and it's confusing on how many ways are there of specyfing this. If we can get rid of teal_data@datanames and stick to ls(get_env(teal_data)) then at least it brings some clarity to the ecosystem. We could go even one step further and rename teal.data::datanames() to teal.data::ls_env() as it's gonna be even more clear

[gogonzo]

Closing this PR in favour of #1334