Clean up rlang&tidyselect usage

[brookslogan]

• Fix use of missing on args-to-be-enquosed.
• Delegate defused arguments idiomatically.

The most impactful instance of the above is in the S3 -> R6 forwarding in epi_archive, and a commit is already available fixing that, but other instances of one or both of the above appear elsewhere.

[dajmcdon]

@brookslogan Can you be more specific about where/when this happens and where changes would be made? Or else self-assign?

[brookslogan]

Here's an example of such cleanup. I can take some time later to try to identify all impacted spots, or may just self-assign later when I have time to actively work on it.

[brookslogan]

The example cleanup above is now part of #101. I still haven't taken a look at any other rlang&tidyselect usage we have (if any).

[brookslogan]

The above bugfix is now merged into main, but other cleanup might still be necessary. This vignette makes some distinctions that might help us identify additional cleanup. (It also provides some reassurance that {{ can be used to forward tidyselections, while ?embrace-operator only mentions data-masking functions.) It also says to document these NSE args in a particular way identifying what kind of NSE args are being used.