Rename slug classes to ai-label

[tay1orjones]

When Slug was renamed to AILabel(#16803), the props and classnames were not included in the rename. This issue is to fix that.

This rename needs to be backwards compatible. The approach we could take could be:

• Add new duplicate classname selectors to the ailabel.scss styles

  - .#{$prefix}--slug .#{$prefix}--slug__button {
  
  + .#{$prefix}--slug .#{$prefix}--slug__button,
  + .#{$prefix}--ai-label .#{$prefix}--ai-label__button { 

• Update classnames within AILabel components to use the new #{$prefix}--ai-label names
• Create a new sub-issue/tasklist item on v12 #15803 for removing these classes as part of v12

Note

On casing: for situations like react where we have to use PascalCase we decided on AILabel

For non-pascalcase situations like classnames I think the codebase convention would be "sausage-case" (or whatever you like to call it 🙃 ): ai-label

[ariellalgilmore]

we decided on using decorator as a general term

[alisonjoseph]

Related issue: #17303

We also want to update the styles so that the AI styles only apply when the child of decorator is the AILabel component.