Add progress bar to concurrent downloads

[yuzawa-san]

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I was looking over #20963 and enjoy the file size ratios (thanks!). I sometimes run on slow connections and would like some progress bars so I can look at quick glance on how the downloads are going. The prior PR's had no unit tests, and I tested this locally. Here is a video:

progress.mp4

I made sure the bar is 20% (1/5th) of the tty and that it is at least 4 characters long.
I used some unicode box drawing glyphs. I suppose we could use = and - if we want full ASCII support, but the checkmark is already not ASCII, so we should be ok. I am open to feedback on the formatting, sizing, and appearance.

[Copilot]

Pull request overview

This PR adds visual progress bars to concurrent downloads in Homebrew, enhancing the user experience by providing real-time visual feedback on download progress. The implementation calculates the percentage of completion for each download and renders it as a Unicode box-drawing progress bar that occupies approximately 20% of the terminal width.

Key changes:

• Added progress bar rendering logic that displays download completion percentage using Unicode characters (━ for completed, ┈ for pending)
• Modified the progress string format to include the progress bar before the phase and size information
• Removed the square bracket notation from the progress display

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[Copilot]

When total_size is 0 or very small, the percentage calculation can exceed 1.0, causing bar_used to be larger than bar_length. This would result in a negative count for bar_pending on line 308, causing an ArgumentError when attempting to multiply a string by a negative number.

Consider clamping the percent value:

percent = if (total_size = downloadable.total_size)
  (fetched_size.to_f / [1, total_size].max).clamp(0.0, 1.0)
else
  0.0
end

[MikeMcQuaid]

Looks great so far! Want to give this a better review and test but won't be until Monday. Maintainers: please don't merge until then, thanks!