Style provides information on the characters to use before and after the
progress bar content (`[` and `]` for the ascii style), as well as the
character for filled step (`#` for ascii style). When supported, it
provides intermediate steps. This is used for unicode style, to show
better precision.
* guix/progress.scm (<progress-bar-style>): New record type.
(ascii-bar-style, unicode-bar-style): New variables.
(progress-bar): Draw progress depending on style. When supported, use
unicode style. Fall back to ascii style.
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