Allow custom style variation classNames

[greatislander]

Is your feature request related to a problem? Please describe.

The block styles variations API introduced in #7362 uses an is-style-{styleName} className to the block for style variants. This works well for core blocks and new blocks, but may be overly opinionated for converting existing content that has an established approach to styling variants (see #7362 (comment)). For example, a pattern library that uses BEM conventions might use .element for a default element, and .element.element--variant for a variant of that element. I'd advocate for a solution that does not require changing an existing naming convention if one exists.

Describe the solution you'd like

A className attribute could be added to each style variation which, if supplied, would be used instead of the default generated className. Here's some pseudocode:

styles: [
    { 
        name: 'default', 
        label: __( 'Rounded' ), 
        isDefault: true 
    },
    { 
        name: 'outline', 
        label: __( 'Outline' ),
        className: 'button--outline' 
    },
    { 
        name: 'squared', 
        label: __( 'Squared' ) 
        className: 'button--squared' 
    },
],

Describe alternatives you've considered

The only alternative at present is to use the opinionated is-style-{styleName} naming convention.

[mrwweb]

Completely agree. The point about converting old code seems especially relevant.

[vicolaspetru]

It's possible to get active style?
If I want to change the paragraph position depending on style, how I can do this?

[mrwweb]

@vicolaspetru The CSS class is applied to the block container both in the admin/editor and front-end, so registering gutenberg editor styles should be all you need to do.

[wp-stars-vienna]

Ist der still no way to remove that 'is-style' prefix? i just need that to use bootstrap classes.

[apsolut]

Please add filter to remove/disable prefixes like: has- , is-style-

[skorasaurus]

Admittedly, requiring to use the is-style prefix is frustrating because we're having to change our naming structure (BEM based) for our growing pattern library/components to match Gutenberg's decisions. :/

[ecdani]

As a workaround you can prepend a whitespace to your class. Then you will have a useless "is-style-" class and the class you want.