Breaking change

• The cli_theme_dark option is know known as cli.theme_dark, to be
  consistent with all other cli option names (#380).

Other changes

• The preferred names of the S3 clases ansi_string, ansi_style, boxx,
  rule and tree now have cli_ prefix: cli_ansi_string, etc. This will
  help avoiding name conflicts with other packages eventually, but for now
  the old names are kept as well, for compatibility.

• cli_abort() has been updated to work nicely with rlang 1.0. The
  default call and backtrace soft-truncation are set to .envir
  (which itself is set to the immediate caller of cli_abort() by
  default).

  Line formatting now happens lazily at display time via
  rlang::cnd_message() (which is called by the conditionMessage()
  method for rlang errors).

• New hash_sha256() function to calculate SHA-256 hashes. New
  hash_raw_*(), hash_obj_*() and hash_file_*() functions to calculate
  various hashes of raw vectors, R objects and files.

• You can use the new cli.default_num_colors option to set the default
  number of ANSI colors, only if ANSI support is otherwise detected.
  See the details in the manual of num_ansi_colors().

• You can set the new ESS_BACKGROUND_MODE environment variable to
  dark to indicate dark mode.

• cli now handles quotes and comment characters better in the semantion
  cli_*() functions that perform glue string interpolation (#370).