Enhancement: Add linkedByDefault spacing setting to theme.json - #84719

What?

Closes #70646

This PR introduces a new theme.json setting, settings.spacing.linkedByDefault, which allows theme developers to determine whether margin and padding controls should have their sides linked or unlinked by default in the block editor.

To ensure strict backward compatibility, this setting defaults to true. It accepts either a global boolean or a granular object ({ margin: boolean, padding: boolean }) for independent control.

Why?

Currently, when a user adds a block and attempts to adjust the margin, the top and bottom margin settings are linked by default, which often results in both margins being changed simultaneously. This contrasts with standard block spacing paradigms where typically only a top margin is applied.

While developers could previously use the register_block_type_args filter to restrict spacing support to only the top side, this aggressively removed the ability for users to edit the other sides. This PR provides a non-destructive UI solution that makes it easier to adjust a single side by default without taking any design tools away from the editor.

How?

• Added linkedByDefault (boolean or object) to theme.json schema and allowlisted it in the PHP and JS style engines.
• Updated DimensionsPanel to fetch the setting and pass strict marginLinked and paddingLinked booleans.
• Updated BoxControl and SpacingSizesControl to default to unlinked when empty, while preserving the linked view for existing uniform attributes.

Testing Instructions

1. In your testing environment, modify your active theme's theme.json to include the new setting. Test both the global boolean and the object override:

   "settings": {
       "spacing": {
           "linkedByDefault": false
       }
   }

2. Alternatively, test the granular object:

"settings": {
    "spacing": {
        "linkedByDefault": {
            "margin": false,
            "padding": true
        }
    }
}

3. Open a post or page in the block editor.
4. Insert a Group block (ensure it has no previously saved spacing values).
5. Open the Dimensions panel in the inspector controls.
6. Verify that the Margin (and/or Padding, depending on your test config) controls render as unlinked by default (showing 4 individual side sliders/inputs).
7. Verify that you can still manually click the "Link sides" button to bind them together.
8. Insert a block that already has uniform spacing values saved to it. Verify that the controls correctly initialize as linked, overriding the linkedByDefault: false preference to match the block's existing data.

Screenshots or screencast