Add support to SF Symbols for Variable Colors (variableValue)

[karillo]

Describe the bug
In addition to SF Symbols support for renderingMode, weight and scale, it would be useful to be able to specify the variableValue for Symbols with variable color support.

Ref: https://developer.apple.com/documentation/uikit/uiimage/3955178-init

To Reproduce
Feature request

Expected behavior
I was hoping that perhaps this came along with the work for sfconfig.

Possible something along the lines of an sfconfig string of:

{"variableValue":0.25}

or a new parameter like

| sfvalue=0.25

Environment:

• macOS version: 14.2.1
• SwiftBar version: 2.0.0

Plugin Example:
The :wifi: symbol supports variable color (variableValue). I was imagining something like this:

#!/bin/bash

WIFIDATA=$(/System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I)
SSID=$(echo "${WIFIDATA}" | awk '/ SSID/ {print substr($0, index($0, $2))}')
SIGNAL=$(echo "${WIFIDATA}" | awk '/ agrCtlRSSI/ {print substr($0, index($0, $2))}')
NOISE=$(echo "${WIFIDATA}" | awk '/ agrCtlNoise/ {print substr($0, index($0, $2))}')

SNR="$((SIGNAL - NOISE))"

QUALITY="$((2 * SNR))"
QUALITY="$((QUALITY < 100 ? QUALITY : 100))"
SFCONFIG='{"variableValue":'$(bc -e "scale=2;${QUALITY}/100")'}'

cat <<EOF
${QUALITY}% | sfimage=wifi sfconfig=$(echo -n "${SFCONFIG}" | base64)
---
$SSID
---
Refresh... | refresh=true
EOF