[Patch Included] Style variation chooser icons don't support non-default weight

[Kirby]

Affected version

2.3.3

js/xf/core.js

const newIcon = XF.createElementFromString(
    XF.Icon.getIcon('default', icon)
)
menuIcon.replaceWith(newIcon)

This code essentialy prevents using style variation switcher icons in a non-default weight as the runtime-generated icon HTML becomes invalid if a diffrent weight (fas, etc.) is specified.

It seems unnecessarily limiting and inconsistent with other parts of the software (like forum type icons) where it is possible to use non-default weights.

The attached patch fixes this by introducing a new method XF.Icon.getIconParamsFromClassString to extract the required info (variant, name, classes) from a given class string and uses this to display the correct icon while also adding support for duotone icons.