Twenty Twenty: Remove/replace abandoned `stylelint-a11y` dependency

[desrosj]

The stylelint-a11y dependency has been abandoned, not receiving an update for over 7 years.

The package is currently incompatible with newer versions of Stylelint and Node.js.

This should be removed or replaced.

[desrosj]

This is blocking the efforts to update Core from Node.js 20.x to 24.x in #65451.

[desrosj]

It looks like there is a ​reasonable replacement in @double-great/stylelint-a11y. However, this would need to make use of version 3.4.15 initially because 3.5.0 drops Node.js 20.x support.

As for how this is used within the theme, only 2 rules are utilized:

• a11y/no-outline-none
• a11y/selector-pseudo-class-focus

Currently, there are no violations of the first, and 2 of the second (though it's actually one true violation within source files):

assets/css/editor-style-block.css
964:1    ✖  Expected that .editor-styles-wrapper .wp-block-button__link:hover, .editor-styles-wrapper .wp-block-file__button:hover is used together with :focus pseudo-class

assets/css/editor-style-block-rtl.css
960:1    ✖  Expected that .editor-styles-wrapper .wp-block-button__link:hover, .editor-styles-wrapper .wp-block-file__button:hover is used together with :focus pseudo-class

Twenty Twenty is the only default theme that has this dependency, or as far as I can tell, an equivalent rule. Another option is to fix this one violation and remove this dependency entirely. While not ideal as it removes a level of protection against introducing bad accessibility patterns, the linting is not currently run as part of the current GitHub Actions workflows. So it's dependent on contributors running the lint:css command locally.

The ​`@double-great/stylelint-a11y` package is a seemingly well maintained fork of the original stylelint-a11y which has not been updated for over 7 years.

This replaces stylelint-a11y with @double-great/stylelint-a11y.

Note: the latest version of the package is not used intentionally. Version 3.5.0 dropped support for Node.js 20.x. After Core is updated to use 24.x, the latest version of @double-great/stylelint-a11y can be used.

Trac ticket: Core-66004.

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Trac ticket: Core-66004.

## Use of AI Tools

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[sabernhardt]

The stylelint-a11y dependency was added in
​https://github.com/WordPress/twentytwenty/pull/626

[54687] added the :hover underline for Button and File blocks in the editor. The button-style links are underlined for the :focus state on the front end, but these elements do not receive focus in the editor.