Add support for layers to the Icon field

Clone of #2933851: Add support for masking to the Icon field for supporting layers

It would be awesome if we could easily use FA layering and masking. https://fontawesome.com/how-to-use/svg-with-js#masking and https://fontawesome.com/how-to-use/svg-with-js#layering

For masking, maybe add a checkbox to the field settings which adds a secondary autocomplete field to use as the mask?

For layers, maybe an option is to make the fontawesome field multi-value ( field storage -> Allowed number of values > 1 ), and if there is more than one field present, change the template to have the .fa-layers class on the wrapper?