CMS: Create custom string selector for block theme

[regisphilibert]

I'd really like to use this tutorial to give editors a better experience when chosing style impacting options.

https://www.sanity.io/guides/create-a-rich-string-selector-field-input

I know we have a solution based on the color set, but this does not give a detailed idea of the upcoming styling (except for one of the used color) and requires to use said color as the value saved.

With the custom component solution we could:

1. present a button styled as a minimal representation of the block itself, which could preview the colors of the title, copy text border etc...
2. Finally train ourselves on custom input component with which we could bring our editors experience to the next level.