Release visdat 0.5.1

[njtierney]

• [x] devtools::check_win_devel()
• [x] rhub::check_for_cran()
• [x] revdepcheck::revdep_check(num_workers = 4)
• [x] Polish NEWS
• [x] If new failures, update email.yml then revdepcheck::revdep_email_maintainers()
• [x] Bump version (in DESCRIPTION and NEWS)
• [x] devtools::check_win_devel() (again!)
• [x] devtools::release()
• [x] Approve email
• [x] pkgdown::build_site()
• [x] Tag release
• [x] Bump dev version
• [ ] Write blog post
• [ ] Tweet

Template from https://github.com/r-lib/usethis/issues/338