Block Editor: Allow a theme to override global defaults for editor features

[gziolo]

Description

Related to #22622.

Follow up for #22291 where initial handling for block editor features were introduced.

Usage:

// edit.js

const Edit = ( props ) => {
    const isDisabled = ! useEditorFeature( 'typography.dropCap' );
    // ...
};

At the moment this feature can be only controlled through the defaults set for the Gutenberg plugin in lib/exeperimental-default-theme.json file in the features section of the global settings. On the block level, it can be controlled through supports.

The idea is to take it to the next level – take & merge data from the global subtree of the experimental-theme.json with the existing data in lib/exeperimental-default-theme.json.

The full proposal from @nosolosw in #22291 (review).