Add roundsize function

[Bouk250]

Description

I added a function for easily pars size value into human-readable value.

Usage

;; RAM Stats Widgets ;;
(defwidget ram_stats [used_mem total_mem]
      (label
        :text `${round(used_mem.size,2)}${used_mem.units}/${round(total_mem.size,0)}${total_mem.units}`)
    )
  )

;; Stats Widgets ;;
(defwidget stats []
    (ram_stats 
    :total_mem {roundsize(EWW_RAM.total_mem)}
    :used_mem {roundsize(EWW_RAM.used_mem)})
  )
)

Showcase

It's help to parse magic variable that provide size in bytes without external script

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

[elkowar]

Not sure I'm a fan of having this return JSON, tbh -- wouldn't just having a "formatbytecount" function or something like that that takes a number of bytes and turns it into a string with the appropriate unit be sufficient?