Elements: Fix instance element styles for links applying to buttons

[aaronrobertshaw]

Syncs the changes from WordPress/gutenberg#59114

These changes fix the selector used by block instance element styles for links so they do not get incorrectly applied to buttons.

To test:

1. Run: npm run test:php -- --filter Tests_Block_Supports_WpRenderElementsSupport
2. Edit a post, add a group block containing a paragraph with a link and a button
3. Select the group block and apply a color to links within that block
4. Save and view the post on the frontend, the button's color should match the editor and not be impacted by the link color previously selected

Trac ticket: https://core.trac.wordpress.org/ticket/60557

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[mukeshpanchal27]

Why not use a:not(.wp-element-button)?

[aaronrobertshaw]

Thanks for the question @mukeshpanchal27, I take it you mean "why not use :not"?

This selector uses the :where to reduce specificity and make it easier for themes to override the link element styles as desired.

In the case of this PR, it's a bug fix that prevents the misapplication of the link element styles to buttons but by using the same selector as theme.json it also avoids increasing the specificity. This keeps the ease of overriding styles if a theme desires while still resulting in the expected behaviour.

The original PR introducing the lowered specificity for link element styles in theme.json can be found in WordPress/gutenberg#42669. That was backported in #3206.

[youknowriad]

Commit https://core.trac.wordpress.org/changeset/57664