feat: add byteshumanreadable(bytes, num_digits) to expressions

[bkueng]

Description

Can be used to display bytes, e.g. free disk space ${byteshumanreadable(EWW_DISK["/"].free,1)} -> 150.3 GB.

Or also for rates, e.g. ${byteshumanreadable(EWW_NET.wlp3s0.NET_DOWN,0)}/s -> 392 KB/s

The name is a bit long, but I could not think of anything better.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• All widgets I've added are correctly documented.
• I added my changes to CHANGELOG.md, if appropriate.
• The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• I used cargo fmt to automatically format all code before committing