Add variables to icon library

[pseys]

To support the potential for icon library swapping in Figma, improvements need to be made.

• Add icon-size variable to height and width of all icons in the library
• Replace Salt-current color style (gray-300) used by all icons to content-primary-foreground variable

This is an outcome of the semantic icon POC #3625

[pseys]

The following variables have been added to the icon library on each icon:

• height: size-icon
• width: size-icon
• color: content-primary-foreground

The updates have been published.

[mark-tate]

Goal: S(3) Published in Figma complete, will be released Wk1 of Zazu