- Remove `useEuiButtonRadiusCSS` in favor of a more agnostic `euiButtonSizeMap` - using the raw data is less strict and generates fewer classNames

- `EuiButtonIcon` will shortly need the height/radius map and does not need to use all of `EuiButtonDisplay`'s logic (e.g. doesn't need fullWidth, or a content/text wrapper)

- Move `_buttonSize` to internal fn to make it unexportable / clearer that its CSS is specific to `EuiButtonDisplay`

… to separate file

+ focus CSS comes from a hook

- this can mostly be simplified down from its Sass mixin to just the cursor CSS

- the loading spinner color comment + logic is copied from `EuiButtonDisplay` - it previously existed in the `euiButtonContentDisabled` Sass mixin, but was targeting a selector that did not exist in EuiButtonIcon

+ convert enum types to new preferred syntax
+ move type enums to top of file

+ configure all playgrounds to sort props by required first, then alphabetical

- preload all icons into the cache at once in the preview

- this increases Storybook's startup time somewhat significantly, but will probably be necessary until we resolve dynamic icons for SSR/Vite