Fix CRAN warning due to S3 conditional registration

[DavisVaughan]

CC @lionel-

Fixes this NOTE seen in CRAN's checks on R-devel (>=4.3.0 at the time).

* checking S3 generic/method consistency ... NOTE
Apparent methods for exported generics not registered:
  vec_cast.data.frame.grouped_df vec_cast.data.frame.rowwise_df
  vec_cast.data.frame.sf vec_cast.data.frame.tbl_df
  vec_cast.grouped_df.data.frame vec_cast.grouped_df.grouped_df
  vec_cast.grouped_df.tbl_df vec_cast.rowwise_df.data.frame
  vec_cast.rowwise_df.rowwise_df vec_cast.rowwise_df.tbl_df
  vec_cast.sf.data.frame vec_cast.sf.sf vec_cast.tbl_df.data.frame
  vec_cast.tbl_df.grouped_df vec_cast.tbl_df.rowwise_df
  vec_cast.tbl_df.tbl_df vec_proxy.sf vec_proxy_order.sfc
  vec_ptype2.data.frame.grouped_df vec_ptype2.data.frame.rowwise_df
  vec_ptype2.data.frame.sf vec_ptype2.data.frame.tbl_df
  vec_ptype2.grouped_df.data.frame vec_ptype2.grouped_df.grouped_df
  vec_ptype2.grouped_df.tbl_df vec_ptype2.rowwise_df.data.frame
  vec_ptype2.rowwise_df.rowwise_df vec_ptype2.rowwise_df.tbl_df
  vec_ptype2.sf.data.frame vec_ptype2.sf.sf vec_ptype2.sf.tbl_df
  vec_ptype2.tbl_df.data.frame vec_ptype2.tbl_df.grouped_df
  vec_ptype2.tbl_df.rowwise_df vec_ptype2.tbl_df.sf
  vec_ptype2.tbl_df.tbl_df vec_restore.grouped_df
  vec_restore.rowwise_df vec_restore.sf
See section 'Registering S3 methods' in the 'Writing R Extensions'
manual.

This is due to some rewriting of the S3 generics/method check system:
r-devel/r-svn@d34f52a

It only affects methods that we conditionally register which aren't actually registered at vctrs load time. i.e. the dplyr methods are conditionally registered:

• When dplyr is eventually loaded
• If dplyr doesn't already have these methods itself (which it doesn't as of dplyr 1.1.1)

I imagine the CRAN check sees that vctrs is "fully loaded" but it didn't officially register S3 methods for these functions that "look" like S3 methods due to the . in their names, and that triggers the note.

Switching from a . to an _ should avoid the NOTE, but also requires manual registration of the method in s3_register().

[DavisVaughan]

I can reproduce on r-devel locally with this new envvar:

devtools::check(env_vars = c(NOT_CRAN = "true", `_R_CHECK_S3_METHODS_SHOW_POSSIBLE_ISSUES_` = "true"))

I have confirmed that with this PR it no longer shows the NOTE.