Update utilities-tidy-eval.R and use as_label()

[yutannihilation]

Close #4063

What I did to create this PR was:

• update utils-tidy-eval.R by usethis::use_tidy_eval()
• add @export quo_name for backward-compatibility, considering we've been exporting it for 2 years with a documentation using it
• replace quo_name() with as_label() in the code and the docs

As a result, this PR changes the following two points:

• adds some new re-exported functions :=, as_name(), and as_label().
• removes some re-exported functions quo(), quos(), enexpr(), enexprs(), ensym(), ensyms(); they are different than quo_name() in that no documentations of ggplot2 refer to them, so I believe it's OK to remove them.

[yutannihilation]

@lionel-
Sorry to bother you. Could you take a look when you have time? I believe this PR is safe to merge, but I want to confirm if this is consistent with the latest direction of rlang things.

[lionel-]

LGTM. I have opened an issue about updating the template because it should now recommend reading the dplyr and ggplot2 programming vignettes: r-lib/rlang#1008

Regarding backward compatibility, it might still cause issues to remove the other operators from the package because they were mentioned in the old dplyr programming vignette or in the tidyeval bookdown. To be safe it would be better to keep them in an @keyword internal topic.

[yutannihilation]

Thanks! OK, let's keep other functions as well.