Update functions to use linelist style tagging

[chartgerink]

This PR reverts the naming of tags -> labels back to tags. In the process, there are also changes to revert to consistency with linelist, and help improve drop-in update on that side. This means that the arguments in this PR are consistent with functions in linelist

Commits:

• Update tags()
• Redocument etc
• Update tag_variables()
• Update drop_safeframe()
• Update restore_tags() and related
• Update square_brackets.R and downstream
• Update make_safeframe()
• Update print.safeframe()
• Ensure tags() returns empty list
• Update set_tags()
• Update validate_tags()
• Update validate_*() functions
• Update type()
• Update tags_df()
• Remove names() (default is sufficient now)
• Update remaining tests
• Update dplyr vignette
• Update has_tag() and safeframe-package
• Regen docs, ensure checks etc run

[chartgerink]

Seems like setdiff has changed since the last release in a way that is altering how the result is returned (see wch/r-source@6dedb30).

Old (r-release):

x <- make_safeframe(cars, mph = 'speed')
tags <- list(mph = "speed", mph = "edad")
setdiff(tags, names(x))

$mph
[1] "speed"

New (r-devel):

x <- make_safeframe(cars, mph = 'speed')
tags <- list(mph = "speed", mph = "edad")
setdiff(tags, names(x))
[[1]]
[1] "edad"

I replaced it with a more consistent implementation for our purposes in b51a9d6