Add support for binding variables to effects

[six7]

Figma recently released support for binding variables to effects (shadows, blur), we should add those as well which would be huge for themable gradients.

https://www.figma.com/plugin-docs/updates/

• When applying a shadow that's referencing a color token and that color token has a variable connected, create the reference inside the shadow effect
• Do the same for background blur

https://www.figma.com/plugin-docs/api/VariableBindableEffectField/