Commands: Add unified API for keyboard shortcuts and commands

[Sukhendu2002]

What?

Part of: #59004

This PR implements Step 1 of the proposal to unify keyboard shortcuts and commands under a single API.

This PR introduces:

• triggerCommand action to programmatically execute a command
• useShortcutCommand hook to bind a keyboard shortcut to a command

Example

// 1. Register the command
useCommand({
    name: 'core/toggle-fullscreen-mode',
    label: isFullscreen ? __( 'Exit fullscreen' ) : __( 'Enter fullscreen' ),
    icon: fullscreen,
    callback: ({ close }) => {
        toggle( 'core/edit-post', 'fullscreenMode' );
        close();
        createInfoNotice(...);
    },
});

// 2. Connect the shortcut to the command
useShortcutCommand( 
    'core/edit-post/toggle-fullscreen',  // Shortcut name
    'core/toggle-fullscreen-mode'        // Command name
);

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