Add `@double-great/stylelint-a11y`

[Mouvedia]

What is the problem you're trying to solve?

stylelint-a11y has been removed in #9.
@ronilaukkarinen/stylelint-a11y is only compatible with 15.x.x.
@double-great/stylelint-a11y supports the last major.

What solution would you like to see?

Replace stylelint-a11y by @double-great/stylelint-a11y.

[ybiquitous]

@double-great/stylelint-a11y also seems unmaintained 🤔

[Mouvedia]

@ybiquitous @double-great/stylelint-a11y had a release last week.

[ybiquitous]

If the package is maintained, I'm okay with adding it.