Core/File: Reduce specificity of pseudo-selector styles for better override support via theme.json

[yogeshbhutkar]

What?

Closes #70342

This PR lowers the specificity of pseudo-selector styles within the .wp-block-file__button class to 0,1,0, allowing them to be more easily overridden via theme.json.

Why?

Previously, the :visited and :hover pseudo-selectors on file block buttons could not be overridden using theme.json due to higher specificity.

How?

Replacing &:is(a) with &:where(a) preserved the styles while reducing specificity from 0,1,1 to 0,1,0.

Testing Instructions

1. Create a new post.
2. Add a file block.
3. Attach a file to it.
4. Use the following code and paste it under the styles key within theme.json.

Code

"core/file": {
  "elements": {
	"button": {
		":hover": {
			"color": {
				"text": "#d34949"
			}
		},
		":visited": {
			"color": {
				"text": "#49d353"
			}
		}
	}
  }
}

5. Confirm that the hover and visited styles are properly applied on the front-end.

Testing Instructions for Keyboard

Same.

Screenshots or screencast

Before	After

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Co-authored-by: yogeshbhutkar <[email protected]>
Co-authored-by: Mukulsingh27 <[email protected]>

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