redo warnings for overwritten arguments in abline, hline, and vline

[paleolimbot]

This is a pull request to address #3090, and in part overwrites the work from #2950. This PR ensures that:

• An informative warning is issued when mapping and one or more of yintercept/xintercept/slope/intercept are provided.
• An informative warning is issued when data and one or more of yintercept/xintercept/slope/intercept are provided.

It is also worth noting that I changed the test for "data is provided" from missing(data) to is.null(data). I am not sure if there was a good reason for it to have been missing(data) in the first place, as something like

data <- NULL
geom_hline(yintercept = 3, data = data)

would possibly not issue a warning.

I may have gone overboard on the gramatically-correct informative warning message function (warn_ovewritten_args()), but was trying make all the messages informative without duplicating code.

[hadley]

Looks good to me. Just needs a NEWS bullet

[paleolimbot]

Should I add the NEWS bullet here? It doesn't look like there's a set of bullet points started for post-3.2.0.9000 and I think the NEWS.md on my branch is out of date.

[thomasp85]

Is this intended for 3.2.0?

[paleolimbot]

No, it wasn't.

[thomasp85]

Ah. Just make a new heading and a bullet then

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