Non Design System variables injected into variable selection list

Describe the Problem When editing an element's styles I am unable to select variables that aren't defined in the Design System. For example, if I wanted to make use of the WordPress variables from global-styles-inline-css For size controls I can use the function option to enter something like var(wp--preset--spacing--20), but that's not possible for color controls, and it's not ideal. Propose a Solution I can image different possibilities, with varying implications. 1. UI: Allow _no value_ as an option in the Design System. In my example, this would allow me to add wp--preset--gradient--luminous-dusk as a variable, making it available for selection when editing an element's Style color control. Having 'no value', Elementor could skip it when generating CSS files. 2. Code: Provide a filter in Variables_Service::load() for adding additional variables. Using this I could add wp--preset--gradient--luminous-dusk to the list of v…