I am in favour of adding this to std.
What should it look like by default and what should be customizable?
There are many different cli progress bar styles in cli land, some with, some without percentage and/or n/m displays, prefixes, suffixes etc.
[####----------------], [####----------------] 20%, [####----------------] 4/20
[###>----------------], [###>----------------] 20%, [###>----------------] 4/20
[===>----------------], [===>----------------] 20%, [===>----------------] 4/20
[■■■■□□□□□□□□□□□□□□□□], [■■■■□□□□□□□□□□□□□□□□] 20%, [■■■■□□□□□□□□□□□□□□□□] 4/20
▓▓▓▓░░░░░░░░░░░░░░░░, ▓▓▓▓░░░░░░░░░░░░░░░░ 20%, ▓▓▓▓░░░░░░░░░░░░░░░░ 4/20

I think it probably should be customizable for the bars and percentage at a minimum. Having it templated would also be nice probably.

I've used both @deno-library/progress and dax for this and both seem to work well for my purposes.

I think by default it should look the same as deno has when updating, but the progress bar characters should be customisable, along with showing an actual percentage and some way to communicate a rate of some sort.

I also use number of steps vs a percentage quite often for the progress labelling. I don't recall what the Deno default is, but I'm somewhat partial to [============] style as a basic/non fancy default.